This specification defines modifications to the existing CSS Fonts 3 specification along with additional features for font feature control, font variations, and chromatic (color) fonts.
CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc.
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and the latest revision of this technical report
can be found in the W3C standards and drafts index.
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1.
Introduction
This specification includes and expands on
the basic font features described in the CSS3 Fonts specification ([CSS-FONTS-3]).
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the CSS-wide keywords as their property value.
For readability they have not been repeated explicitly.
2.
Basic Font Properties
The particular font face used to render a character is determined by
the font family and other font properties that apply to a given element.
This structure allows settings to be varied independently of each
other.
This property specifies a prioritized list of font family names or generic family names.
A font family defines a set of faces that vary in weight, width or slope.
CSS uses the combination of a family name with other font properties to select an individual face.
Using this selection mechanism,
rather than selecting a face via the style name as is often done in design applications,
allows some degree of regularity in textual display when fallback occurs.
Component values are a comma-separated list indicating alternatives.
A user agent iterates through the list of family names
until it matches an available font
that contains a glyph for the character to be rendered.
(See § 5.3 Cluster matching.)
This allows for differences in available fonts across platforms and
for differences in the range of characters supported by individual fonts.
body
Helvetica Verdana sans-serif
If Helvetica is available, it will be used when rendering.
If neither Helvetica nor Verdana is present,
then the generic font-family sans-serif font will be used.
There are three types of font family names, two of which may be used with this property:
Each <generic-font-family> keyword represents
a generic font choice,
and behaves as a potential alias for one or more locally-installed fonts
belonging to the specified generic font category.
A <generic-font-family> can thus be used as a fallback
for when an author’s more specific font choices are not available.
There are three types of generic family:
Generics which apply to all Unicode characters
and will always match a locally installed font.
For example, monospace.
Generics which apply to all Unicode characters
but may not match to a locally installed font
on some systems.
For example, ui-rounded.
Generics which are writing-system specific,
only apply to a subset of Unicode characters,
and may not match to a locally installed font
on some systems.
For example, generic(fangsong).
Authors are encouraged to append a generic font family as a last alternative
for improved robustness,
and to use a more specific generic font family, if applicable,
to prefer a specific style even if the individual named font family is not available.
A locally installed system font, whose use is subject to certain constraints.
In particular, it may not be used with the font-family property,
but can be used with the font shorthand.
The following values refer to system fonts:
caption
The font used for captioned controls (e.g., buttons, drop-downs, etc.).
icon
The font used to label icons.
menu
The font used in menus (e.g., dropdown menus and menu lists).
Font family names other than generic families or system font families
must either be given quoted as <string>s,
or unquoted as a sequence of one or more <custom-ident>.
Note: This means most punctuation characters and digits at the start of
each token must be escaped in unquoted font family names.
To illustrate this, the following declarations are invalid:
Red/Black sans-serif
Grande sans-serif
Ahem! sans-serif
test sans-serif
#POUND sans-serif
Hawaii sans-serif
Any identifier
which could be misinterpreted
as a pre-defined keyword in the font-family value definition,
or the CSS-wide keywords,
is not allowed.
Note: this means that if you really have a font
whose name
is the same as one of the <generic-font-family> names,
or the system font names,
or the CSS-wide keywords,
it must be quoted.
To illustrate this,
the following unusual font family names are valid
because they are quoted:
sans-serif
sans-serif
sans-serif
sans-serif
If a sequence of identifiers is given as a <font-family-name>,
the computed value is the name
converted to a string
by joining all the identifiers in the sequence by single spaces.
To avoid mistakes in escaping,
it is recommended to quote font family names
that contain white space, digits,
or punctuation characters other than hyphens:
Quoting font families prevents escaping mistakes.
body serif
<body style=>
Font family names that happen to be the same as
a font-family keyword value
(e.g. CSS-wide keywords such as inherit, or
<generic-font-family> keywords such as serif)
must be quoted to prevent confusion
with the keywords of the same names.
UAs must not consider these keywords as matching the <font-family-name> type.
To make the syntax less succeptible to clashes, more recently defined generic font families are identified using a functional syntax.
body fangsong serif
The first choice in this example is a specific, named font, in Fang Song (仿宋) style.
The family name is enclosed in quotes as it contains space characters.
The second is a recently added script-specific generic font; it is unicode-range specific
and so may not match to an actual installed font on some systems;
but if it exists it will be an example of the requested style.
The third is a universal generic font, which is guaranteed to match on all systems.
A font family name only specifies a name given to a set of font faces;
it does not specify an individual face.
For example, given the availability of the fonts below,
Futura would match but Futura Medium would not:
👁 family and face names Family and individual face names
Note: The CSS definition of font attributes used for selection
are explicitly not intended to define a font taxonomy.
A type designer’s idea of a family can often extend
to a set of faces that vary along axes other than just
the standard axes of
weight (font-weight),
width (font-width),
and slant (font-style).
A family can vary along axes that are unique to that family.
The CSS font selection mechanism merely
provides a way to determine the “closest” match
when substitution is necessary.
Note: The precise way a set of fonts are grouped into font families
varies depending upon the platform font management APIs.
For example, the Windows GDI API only allows four faces to be grouped into a family,
while the DirectWrite API,
Core Text API,
and other platforms support
font families with a variety of weights, widths, and slopes
(see Appendix A: Mapping platform font properties to CSS properties for more details).
A generic font family is a font family which has a standard name (as defined by CSS), but which is an alias for an existing installed font family present on the system.
However, a single generic font family may be a composite face
combining different typefaces based on such things as
the Unicode range of the character,
the content language of the containing element,
user preferences, system settings, etc.
Different generic font families may map to the same used font.
Note: Generic font families are intended to be widely implemented on many platforms, unlike arbitrary <font-family-name>s which are usually platform-specific names. They are expected to map to different fonts on different platforms. Authors may specify these generic family names if they desire their text to follow a particular design on many platforms, and are not particular about which specific font is chosen on those platforms.
User agents should provide reasonable default choices for the generic font families,
that express the characteristics of each family as well as possible,
within the limits allowed by the underlying technology.
User agents are encouraged to allow users to select alternative faces for the generic font families.
serif
Serif fonts represent
glyphs that have finishing strokes,
flared or tapering ends,
or have actual serifed endings (including slab serifs).
Serif fonts are typically proportionately-spaced.
They often display a greater variation between thick and thin strokes
than fonts from the sans-serif generic font family.
Note:serif and sans-serif
only apply to a small handful of writing scripts.
Their use as generic font families in CSS is historical,
and reflects the Latin-centric nature
of early Web development.
Better and more widely applicable names would have been,
for example, "modulated" and "monoline".
However, for reasons of Web compatibility,
these names cannot be changed.
CSS uses the term "serif" to apply to a font for any script,
although other names might be more familiar for particular scripts,
such as Mincho (Japanese),
Sung or Song (Chinese),
Batang (Korean).
For Arabic, the Naskh style would correspond to serif.
serif must always map to at least one matched font face.
Note: No guarantee is placed on the character coverage of that font
face. Therefore, the font serif is mapped to may not end up being
used for all content.
Glyphs in sans-serif fonts,
as the term is used in CSS,
are generally low contrast
(vertical and horizontal stems have the close to the same thickness)
and have stroke endings that are plain
(without any flaring, cross stroke, or other ornamentation).
Sans-serif fonts are typically proportionately-spaced.
They often have little variation between thick and thin strokes,
compared to fonts from the serif family.
CSS uses the term "sans-serif" to apply to a font for any script,
although other names might be more familiar for particular scripts,
such as Gothic (Japanese),
Hei (Chinese),
or Gulim (Korean).
sans-serif must always map to at least one matched font face.
Note: No guarantee is placed on the character coverage of that font
face. Therefore, the font sans-serif is mapped to may not end up being
used for all content.
Glyphs in cursive fonts generally use a script style,
and the result looks more like handwritten pen or brush writing than printed letterwork.
CSS uses the term "cursive" to apply to a font for any script,
although other names such as Chancery, Brush, Swing and Script are also used in font names.
👁 sample cursive fonts Sample cursive fonts
fantasy
Fantasy fonts are primarily decorative or expressive fonts
that contain decorative or expressive representations of characters.
These do not include Pi or Picture fonts which do not represent actual characters.
👁 sample fantasy fonts Sample fantasy fonts
monospace
The sole criterion of a monospace font is that all glyphs have the same fixed width.
This is often used to render samples of computer code.
monospace must always map to at least one matched font face.
Note: No guarantee is placed on the character coverage of that font
face. Therefore, the font monospace is mapped to may not end up being
used for all content.
This generic font family lets text render with the default user interface font
(which, as with all § 2.1.5 Generic font families, may be a composite font)
on the platform on which the UA is running.
A cross-platform UA should use different fonts on its different supported platforms.
The purpose of system-ui is to allow web content
to integrate with the look and feel of the native OS.
This is useful for hybrid, web or cross-platform applications.
The precise font(s) used will depend on factors such as
platform support, platform language, locale settings,
user preferences, Unicode coverage and the content language.
UA may optionally choose to use the text rendering engine provided by the platform when using
system-ui to match the appearance of the default user interface more closely.
As with other generic font families,
the substitution of specific installed fonts for system-ui
does not affect the computed style.
<div id= style=></div>
...
window.document..
The script above should not have any knowledge of how system-ui
is expanded to include a collection of system user interface fonts.
In particular, the above script should yield a result of "system-ui" on every platform.
Note:system-ui is highly dependent on the platform environment as multiple factors may
affect the default user interface font, and may have verying level of text rendering support.
This may cause the displayed typeface to be undesirable for some end users
(e.g. Simplified Chinese users might get a monospaced serif font on older version of Windows;
lang attribute might not affect displayed font in cases like lang="uk" in Russian locale
or lang="ja" in Chinese locale).
Some operating system vendors does not offer means to customise system-ui,
unlike UAs that generally allow users to customise the generic sans-serif or serif font family.
As the name implies, system-ui is intended for use with UI elements in web application, and not for large paragraphs of text or articles.
math
This font family is intended for use with mathematical expressions.
Such fonts may include additional data
(for example, the OpenType MATH table)
to help with the hierarchical process of equation layout.
In particular, they may contain
stylistic and stretching glyph variants
useful in setting mathematical equations.
generic(fangsong)
This font family is used for Fang Song (仿宋) typefaces in Chinese.
Fang Song is a relaxed, intermediate form
between Song (serif) and Kai (generic(kai)).
Typically, the horizontal lines are tilted,
the endpoint flourishes are smaller,
and there is less variation in stroke width,
compared to a Song style.
Fang Song is often used for official Chinese Government documents.
Note:generic(fangsong) might not map to any locally installed font,
but if it does, that font will be in Fang Song style.
👁 Image Chinese text displayed with sample fang song fonts
generic(kai)
This font family is used in
Simplified & Traditional Chinese.
A major typeface, which provides calligraphic styles for Chinese text.
It shows notable handwriting features.
Kai is commonly used in official documents and textbooks.
Most official documents in Taiwan use Kai in full text.
Kai can also be combined with other typefaces
to be used in text that needs to be differentiated
from the rest of the content,
for example, headlines, references, quotations, and dialogs.
Note:generic(kai) might not map to any locally installed font,
but if it does, that font will be in Kai style.
👁 Image Chinese text displayed with sample kai fonts
generic(khmer-mul)
This font family is used in
the Khmer language (used in Cambodia),
for titles and headings,
or to emphasise important names or nouns.
Compared to the upright (âksâr chôr) style,
which is used for regular text,
âksâr mul is characterized by
heavier, more rounded letter shapes,
and may include more ligated forms.
Despite the visually heavier style,
the font weight for regular text in
generic(khmer-mul) is unchanged (400).
Note:generic(khmer-mul) might not map to
any locally installed font,
but if it does, that font will be in âksâr mul style.
👁 Image Cambodian text displayed in:
(top) âksâr chôr style, set in Khmer OS Battambang;
(center) âksâr mul style, set in Khmer OS Moul v6, with no ligatures;
(bottom) âksâr mul style, set in Khmer OS Moul v4, with two ligatures. All fonts by Dan Hong.
generic(nastaliq)
This font family is the standard way of writing Urdu and Kashmiri,
and is also often a preferred style for Persian and other language text,
especially in literary genres such as poetry.
Key features include a sloping baseline for joined letters,
and overall complex shaping and positioning for base letters and diacritics alike.
There are also distinctive shapes for many glyphs and ligatures.
It is important not to fall back to a naskh style
for languages such as Urdu and Kashmiri.
Note:generic(nastaliq) might not map to any locally installed font,
but if it does, that font will be in Nastaliq style.
👁 السلام علیکم Urdu text displayed with a sample nastaliq font (Noto Nastaliq Urdu)👁 السلام علیکم Urdu text displayed with a fallback naskh font (Scheherazade New)
ui-serif
This font family is used for the serif variant of the system’s
user interface. The purpose of ui-serif is to allow web
content to integrate with the look and feel of the native OS.
Note:ui-serif is not expected to map to any font on platforms
without an appropriate system font.
Note: see notes on system-ui for potential issues.
This font family is used for the sans-serif variant of the system’s
user interface. The purpose of ui-sans-serif is to allow web
content to integrate with the look and feel of the native OS.
Note:ui-sans-serif is not expected to map to any font on platforms
without an appropriate system font.
Note: see notes on system-ui for potential issues.
This font family is used for the monospaced variant of the
system’s user interface. The purpose of ui-monospace is to
allow web content to integrate with the look and feel of the
native OS.
Note:ui-monospace is not expected to map to any font on platforms
without an appropriate system font.
Note: see notes on system-ui for potential issues.
This font family is used for the rounded variant of the system’s
user interface. The purpose of ui-rounded is to allow web
content to integrate with the look and feel of the native OS.
Note:ui-rounded is not expected to map to any font on platforms
without an appropriate system font.
Note: see notes on system-ui for potential issues.
Each number indicates a weight that is at least as dark as its predecessor.
Only values greater than or equal to 1, and less than or equal to 1000, are valid,
and all other values are invalid.
Numeric values typically correspond to the commonly used weight names below.
100 - Thin
200 - Extra Light (Ultra Light)
300 - Light
400 - Normal
500 - Medium
600 - Semi Bold (Demi Bold)
700 - Bold
800 - Extra Bold (Ultra Bold)
900 - Black (Heavy)
Note: A font might internally provide its own weight name mappings,
but those mappings within the font are disregarded in CSS.
Font formats that use a scale other than a nine-step scale
should map their scale onto the CSS scale
so that 400 roughly corresponds with a face that would be labeled as Regular, Book, Roman
and 700 roughly matches a face that would be labeled as Bold.
Alternately, weights may be inferred from style names
that correspond roughly with the scale above.
The scale is relative,
so a face with a larger weight value must never appear lighter.
If style names are used to infer weights,
care should be taken to handle variations in style names across locales.
2.2.1.
Relative Weights
Specified values of bolder and lighter indicate weights
relative to the weight of the parent element.
The computed weight is calculated based on the inherited font-weight value
using the chart below.
Inherited value (w)
bolder
lighter
w < 100
400
No change
100 ≤ w < 350
400
100
350 ≤ w < 550
700
100
550 ≤ w < 750
900
400
750 ≤ w < 900
900
700
900 ≤ w
No change
700
Note: The table above is equivalent to selecting the next relative bolder or lighter face,
given a font family containing normal and bold faces
along with a thin and a heavy face.
Authors who desire finer control over the exact weight values used for a given element
can use numerical values instead of relative weights.
2.2.2.
Missing weights
Quite often there are only a few weights available for a particular font family.
When a weight is specified for which no face exists, a face with a nearby weight is used.
In general, bold weights map to faces with heavier weights and light weights map to faces with lighter weights.
(See the § 5 Font Matching Algorithm for a precise definition.)
Most user agents model a font as having a particular weight
which often corresponds to one of the numbers in the nine-step scale
described above.
While this is true of most fonts, some fonts might be configurable so as to support a range of weights.
In this situation, the user agent uses a face with a weight as close as possible to the weight requested
(see § 5 Font Matching Algorithm for the precise algorithm).
In particular, a user agent using a font which supports a range of weights
should behave the same as if a font is present at each individual weight in the range.
For TrueType / OpenType fonts that use variations,
the wght variation is used to implement varying weights.
Fractional weights are valid.
Although the practice is not well-loved by typographers,
bold faces are often synthesized by user agents for families that lack actual bold faces.
For the purposes of font matching,
these faces must be treated as if they exist within the family.
Authors can explicitly avoid this behavior by using the font-synthesis property.
The font-width property selects a normal,
condensed, or expanded face from a font family.
Values are specified either as percentages
or as keywords which map to a percentage
as defined in the following table:
Absolute keyword value
Numeric value
ultra-condensed
50%
extra-condensed
62.5%
condensed
75%
semi-condensed
87.5%
normal
100%
semi-expanded
112.5%
expanded
125%
extra-expanded
150%
ultra-expanded
200%
<percentage [0,∞]>
values represent the fractional width of the glyphs,
with 100% representing “normal” glyph widths
(as defined by the font designer).
Values less than 0% are invalid.
When a face does not exist for a given width,
values less than 100% map to a narrower face if one exists, otherwise a wider face.
Conversely, values greater than or equal to 100% map to a wider face if one exists, otherwise a narrower face.
Some fonts might support a range of width values;
if the requested width value is not available in the font,
the closest supported value is used, using the same mapping rules
(see the § 5 Font Matching Algorithm for the precise algorithm).
For TrueType / OpenType fonts that support variations,
the wdth variation is used to implement varying widths.
The figure below shows how nine font-width property settings
affect font matching for a font family containing a variety of discrete widths.
Grey indicates a width for which no face exists and a different width is substituted:
👁 width mappings for a family with condensed, normal and expanded faces Width mappings for a font family with condensed, normal and expanded width faces
getComputedStyle() always serializes its value as a <percentage>, regardless of how the value was specified by the author, or whether or not a keyword happens to map to the value.
2.3.1.
Font width: the font-stretch legacy name alias
For historical reasons,
a font-stretch property exists
which is a legacy name alias
and functions in the identical way to
the font-width.
For example, here the legacy font-stretch is used
on level one headings.
h1 condensed
The specified value of the font-width
on those headings
becomes set to condensed'.
For example, here the font-width is used
on level one headings.
User agents must not synthesize condensed or expanded faces
for font families which lack such faces
and which do not have a width variation axis.
In particular, user agents must not geometrically stretch such faces.
by computed value type; normal animates as oblique 0deg
The font-style property allows italic or oblique faces to be selected.
Italic forms are generally cursive in nature while oblique faces are typically sloped versions of the regular face.
Compare the artificially sloped renderings of Palatino "a" and Baskerville "N" in grey
with the actual italic versions:
👁 artificial sloping vs. real italics Artificial sloping versus real italics
Values have the following meanings:
normal
Matches against a face that is classified as a normal face,
one that is neither italic or obliqued.
This represents an oblique value of "0".
italic
Matches against a font that is labeled as an italic face,
or an oblique face if one does not exist.
The angle and direction of slant is unspecified.
left
Matches against a font that is labeled as an italic face,
with a positive (clockwise) slant;
or an oblique face with positive slant, if one does not exist.
right
Matches against a font that is labeled as an italic face,
with a negative (counter-clockwise) slant;
or an oblique face with negative slant, if one does not exist.
Controls matching against an oblique face.
Positive angles represent a clockwise slant;
negative angles represent a counter-clockwise slant.
The lack of an <angle> represents 14deg.
(Note that a font might internally provide its own mapping for "oblique",
but the mapping within the font is disregarded.)
Fractional and negative values are accepted; however,
values less than -90deg or values greater than 90deg are invalid.
If no oblique faces exist,
and font-synthesis-style has the value auto,
a synthetic oblique face will be generated.
👁 negative slant An italic face with negative slant.
The font is Mf Autumn Breeze,
by Misti Hammers.
A font family might contain no italic or oblique faces,
only an italic face and no oblique,
only an oblique face and no italic,
both an oblique and an italic,
multiple oblique faces at various angles,
or various combinations thereof.
The font matching routine will select a font to use
which is of the correct sign as the requested angle,
and (if there are multiple such faces) closest to the requested angle.
In general, for a requested angle greater or equal to 11deg,
larger angles are preferred;
otherwise, smaller angles are preferred.
(See § 5 Font Matching Algorithm.)
For TrueType / OpenType fonts that use variations,
the slnt variation is used to implement oblique values,
and the ital variation with a value of 1 is used to implement the italic values.
Note: the OpenType slnt axis is defined
with a positive angle meaning a counter-clockwise slant,
the opposite direction to CSS.
The CSS implementation will take this into account
when using variations
to produce oblique faces.
What direction should positive and negative obliques skew in vertical writing mode?
How do we achieve skews in the opposite dimension
(needed for vertical writing)?
If no italic or oblique face is available,
oblique faces may be synthesized by rendering non-obliqued faces
with an artificial obliquing operation.
The use of these artificially obliqued faces
can be disabled using the font-synthesis property.
Note: While oblique faces can be simulated by artificially sloping the glyphs of the regular face,
this is not equivalent to a true oblique,
in which optical stroke thicknesses are properly preserved despite the slant.
It is always better to use an actual oblique font rather than rely on a synthetic version.
For the purposes of font matching,
User agents may treat italic as a synonym for oblique.
For user agents that treat these values distinctly,
synthesis must not be performed for italic.
Note: Authors should also be aware that synthesized approaches might not be suitable
for scripts like Cyrillic, where italic forms are very different in shape.
It is always better to use an actual italic font rather than rely on a synthetic version.
Note: Many scripts lack the tradition of mixing a cursive form within text rendered with a normal face.
Chinese, Japanese and Korean fonts almost always lack italic or oblique faces.
Fonts that support a mixture of scripts
will sometimes omit specific scripts, such as Arabic,
from the set of glyphs supported in the italic face.
User agents should be careful about making character map assumptions across faces
when implementing synthesis across fonts,
as italic faces in a family can have different character maps than Roman faces.
This property indicates the desired height of glyphs from the font.
For scalable fonts, the font-size is a scale factor applied to the EM unit of the font.
For non-scalable fonts, the font-size is converted into absolute units
and matched against the declared font-size of the font,
using the same absolute coordinate space for both of the matched values.
Note: There is no requirement on how closely a glyph should fit its EM box.
Individual fonts can have a different apparent visual size
when rendered at the same font-size.
Additionally, glyphs might render arbitrarily far outside their EM box;
and if they overflow their containing block
can induce ink overflow.
A <relative-size> keyword is interpreted
relative to the computed font-size of the parent element
and possibly the table of font sizes. Possible values are:
If the parent element has a keyword font size in the absolute size keyword mapping table,
larger may compute the font size to the next entry in the table,
and smaller may compute the font size to the previous entry in the table.
For example, if the parent element has a font size of font-size:medium,
specifying a value of larger may make the font size of the child element font-size:large.
Instead of using next and previous items in the previous keyword table,
User agents may instead use a simple ratio
to increase or decrease the font size
relative to the parent element.
The specific ratio is unspecified,
but should be around 1.2–1.5.
This ratio may vary across different elements.
Note: A sight-impaired user may request a user agent use a higher ratio than default,
in order to aid readability.
In addition, a user agent may choose to use different ratios
when it detects paragraph text as opposed to title text.
The following style sheet demonstrates
various ways of specifying the font size.
p
blockquote larger
em
em
Note: The used value of this property
can differ from its computed value
due to font-size-adjust.
However, child elements inherit the computed font-size value,
which is not affected by font-size-adjust
(otherwise, the effect of font-size-adjust would compound).
The actual value of this property can differ from the used value
due to the unavailability of certain font sizes.
font-size can be clamped, for responsive typography.
...
2.5.1.
Absolute Size Keyword Mapping Table
The following table provides user agent guidelines for the absolute-size scaling factor
and their mapping to HTML heading and absolute font-sizes.
The medium value is used as the reference middle value.
The user agent may fine-tune these values for different fonts or different types of display devices.
Note: In CSS1, the suggested scaling factor between adjacent indexes was 1.5,
which user experience proved to be too large.
In CSS2, the suggested scaling factor for computer screen between adjacent indexes was 1.2
which still created issues for the small sizes.
The new scaling factor varies between each index to provide a better readability.
To preserve readability, an UA applying these guidelines
should nevertheless avoid creating font sizes
of less than 9 device pixels per EM unit.
For any given font size, the apparent size and effective legibility of text
varies across fonts.
For bicameral scripts such as Latin or Cyrillic that
distinguish between upper and lowercase letters,
the relative height of lowercase letters
compared to their uppercase counterparts
is a determining factor of legibility.
This is commonly referred to as the aspect value
and is equal to the x-height of a font
divided by the font size.
Note: For text which uses diacritics,
too large an x-height will actually decrease legibility
as the diacritics become cramped.
In situations where font fallback occurs,
fallback fonts might not share the same aspect value
as the desired font family
and will thus be less readable.
The font-size-adjust property is a way
to preserve the readability of text when font fallback occurs.
It does this by adjusting the font-size
so that the x-height is the same
regardless of the font used.
The style defined below defines Verdana as the desired font family,
but if Verdana is not available Futura or Times will be used.
One paragraph also has font-size-adjust specified.
p
Verdana Futura Times
p.adj
<p>Lorem ipsum dolor sit amet ...</p>
<p class=>Lorem ipsum dolor sit amet ...</p>
Verdana has a relatively high aspect value of 0.545,
meaning lowercase letters are relatively tall
compared to uppercase letters,
so at small sizes text appears legible.
Times has a lower aspect value of 0.447,
and so if fallback occurs,
the text will be less legible at small sizes than Verdana
unless font-size-adjust is also specified.
How text rendered in each of these fonts compares is shown below,
the columns show text rendered in Verdana, Futura and Times.
The same font-size value is used across cells
within each row and red lines are included to show the differences in x-height.
In the upper half, each row is rendered in the same font-size value.
The same is true for the lower half,
but in this half the font-size-adjust property is also set
to 0.545,
so that the actual font size is adjusted
to preserve the x-height of Verdana across each row.
Note how small text remains relatively legible across each row in the lower half.
This property allows authors to specify an aspect value for an element
that will effectively preserve the x-height of the first choice font,
whether it is substituted or not.
Values have the following meanings:
Specifies the aspect value used in the calculation below
to calculate the adjusted font size:
c = a / a' s
where:
s = font-size value
a = aspect value as specified by the property
a' = aspect value of actual font
c = adjusted font-size to use
Negative values are invalid.
This value applies to any font that is selected
but in typical usage it should be based on the aspect value
of the first font in the font-family list.
If this is specified accurately,
the a/a' term in the formula above
is effectively 1 for the first font
and no adjustment occurs.
If the value is specified inaccurately,
text rendered using the first font in the family list
will display differently in older user agents
that don’t support font-size-adjust.
The value of font-size-adjust affects the used value of font-size
but does not affect the computed value.
It affects the size of relative units
that are based on font metrics
such as ex and ch
but does not affect the size of em units.
Since numeric values of line-height
refer to the computed size of font-size,
font-size-adjust does not affect the used value of line-height.
Note: In CSS, authors often specify line-height
as a multiple of the font-size.
Since the font-size-adjust property affects the used value of font-size,
authors should take care setting the line height
when font-size-adjust is used.
Setting the line height too tightly can result in
overlapping lines of text in this situation.
Authors can calculate the aspect value for a given font
by comparing spans with the same content
but different font-size-adjust properties.
If the same font-size is used, the spans will match
when the font-size-adjust value is accurate for the given font.
Two spans with borders are used to determine the aspect value of a font.
The font-size is the same for both spans
but the font-size-adjust property is specified only for the right span.
Starting with a value of 0.5,
the aspect value can be adjusted
until the borders around the two letters line up.
p
Futura
span
solid red
.adjust
<p><span>b</span><span class=>b</span></p>
The box on the right is a bit bigger than the one on the left, so the aspect value of this font is something less than 0.5.
Adjust the value until the boxes align.
The syntax of this property is based on a traditional
typographical shorthand notation
to set multiple properties related to fonts.
For historical and backwards-compatibility reasons,
it is almost a shorthand property.
Values for the font-variant property can also be included
but only those supported in CSS 2.1;
none of the font-variant values added in
CSS Fonts Levels 3 or 4
can be used in the
font shorthand:
Values for the font-width property can also be included
but only those supported in CSS Fonts level 3,
none of the font-width values added in this specification
can be used in the font shorthand:
All subproperties of the font property
in the Set Explicitly and Reset Implicitly groups
are first reset
to their initial values.
Then, those properties the Set Explicitly group
that are given explicit values
in the font shorthand
are set to those values.
For a definition of allowed and initial values,
see the individual longhand property definitions.
p / sans-serif
p sans-serif
p x-large/ serif
p bold italic large Palatino serif
p normal small-caps / fantasy
p condensed oblique serif
p condensed oblique serif
In the second rule, the font size percentage value ("80%")
refers to the computed font-size of the parent element.
In the third rule, the line height percentage ("110%")
refers to the font size of the element itself.
The first three rules do not specify
the font-variant and
font-weight explicitly,
so these properties
receive their initial values (normal).
Notice that the font family name "new century schoolbook",
which contains spaces,
is enclosed in quotes.
The fourth rule sets the font-weight to bold,
the font-style to italic,
and implicitly sets font-variant to normal.
The seventh rule sets font-style to oblique 25deg,
font-weight to 753, and
font-width to condensed.
Note that the 25deg in this rule must be
immediately following the "oblique" keyword.
Since the font-width property was not defined in CSS 2.1,
when using font-width values within font rules,
authors should include a extra version compatible with older user agents:
p
sans-serif
font: condensed sans-serif
System fonts can only be set as a whole;
that is, the font family, size, weight, style, etc.
are all set at the same time.
These values can then be altered individually if desired.
If no font with the indicated characteristics exists
on a given platform,
the user agent should either intelligently substitute
(e.g., a smaller version of the caption font might be used
for the small-caption font),
or substitute a user agent default font.
As for regular fonts,
if, for a system font, any of the individual properties
are not part of the operating system’s available user preferences,
those properties
should be set to their initial values.
That is why this property is "almost" a shorthand property:
system fonts can only be specified with this property,
not with font-family itself,
so font
allows authors to do more than the
sum of its subproperties.
However, the individual properties
such as font-weight
are still given values
taken from the system font,
which can be independently varied.
Note that the keywords used for the system fonts listed above
are only treated as keywords
when they occur in the initial position,
in other positions the same string is treated
as part of the font family name:
menu
font: large menu
button italic / sans-serif
button p menu
button p em bolder
If the font used for dropdown menus
on a particular system
happened to be, for example,
9-point Charcoal, with a weight of 600,
then P elements that were descendants of BUTTON
would be displayed as if
this rule were in effect:
button p Charcoal
Because the font shorthand
resets to its initial value
any property not explicitly given a value,
this has the same effect as
this declaration:
button p
normal
normal
normal
Charcoal
2.8.
Controlling synthetic faces
Synthetic font faces are a fallback strategy
to provide an approximation to the requested font
when a font family lacks a suitable face.
For example, a user agent might:
synthesize a bold face by drawing a thin stroke around each glyph
synthesize small-caps, superscripts, or subscripts by
rendering regular glyphs at a smaller size and positioning them appropriately
synthesize an oblique face by geometrical shearing of each glyph
Synthesis is a fallback strategy
because the end result,
while perhaps better than nothing at all,
does not meet the level of quality
of a face designed for the purpose.
Variable fonts,
where the font designer has provided
one or more axes of variation
do not count as font synthesis
and their use is not affected by the font-synthesis property.
This property controls whether user agents are allowed
to synthesize oblique font faces
when a font family lacks oblique faces.
auto
Synthesis of oblique faces is allowed
none
Synthesis of oblique faces is not allowed
oblique-only
Synthesis of oblique faces is allowed,
but they must not be used as fallback
if italic is specified
In vertical text,
for positive oblique angles,
the glyph is skewed
such that the line-over edge shifts towards the line-right side
while the line-under edge shifts towards the line-left side.
For negative oblique angles,
the glyph is skewed
such that the line-over edge shifts towards the line-left side
while the line-under edge shifts towards the line-right side.
Skewing is about the center of the glyph.
This property controls whether user agents are required to synthesize
superscript and subscript glyphs for font-variant-position
when a font lacks them.
auto
Synthesis of superscript and subscript forms is required
none
Synthesis of superscript and subscript forms is not allowed
2.8.5.
Controlling synthetic faces: the font-synthesis shorthand
The style rule below disables the use of synthetically obliqued Arabic:
*:langar none
3.
Font Rendering Controls
3.1.
Introduction to Font Rendering Controls
When using downloadable Web Fonts via @font-face,
the user agent needs to know what to do while the font is actively loading.
Most web browsers have adopted some form of timeout:
Browser
Timeout
Fallback
Swap
Chrome 35+
3 seconds
yes
yes
Opera
3 seconds
yes
yes
Firefox
3 seconds
yes
yes
Internet Explorer
0 seconds
yes
yes
Safari
3 seconds
yes
yes
Chrome and Firefox have a 3 second timeout after which the text is shown with the fallback font.
Eventually, a swap occurs:
the text is re-rendered with the intended font once it becomes available.
Internet Explorer has a 0 second timeout which results in immediate text rendering:
if the requested font is not yet available,
fallback is used,
and text is rerendered later once the requested font becomes available.
While these default behaviors are reasonable,
they’re unfortunately inconsistent across browsers.
Worse, no single approach is sufficient to cover the range of use-cases
required by modern user-experience– and performance–conscious applications.
The Font Loading API [CSS-FONT-LOADING-3] allows a developer to override some of the above behaviors,
but that requires scripting,
a non-trivial amount of effort,
and ultimately doesn’t provide sufficient hooks to cover all reasonable cases.
Additionally,
the developer needs to either inline the loading script into their page
or load an external library,
introducing additional network latency before the fonts can be loaded
and delaying text rendering.
Design/performance-conscious web developers have a good sense
for the relative importance of a given Web Font
for the intended user experience.
This specification provides them the ability to control
font timeout and rendering behavior.
Specifically, it lets developers:
Define the font display policy when
text is ready to be painted:
block, or paint with fallback.
Define the font display policy once
the desired font is available:
rerender text with the new font,
or leave it with the fallback.
Define custom timeout values for each font.
Define custom display and timeout policies per element.
3.2.
The Font Display Timeline
At the moment the user agent first attempts
to use a given downloaded font face on a page,
the font face’s font download timer is started.
This timer advances through three periods of time
associated with the font face—the block period,
the swap period, and
the failure period—which dictate the rendering behavior of any elements using the font face:
The first period is the font block period.
During this period,
if the font face is not loaded,
any element attempting to use it must instead
render with an invisible fallback font face.
If the font face successfully loads during the block period,
the font face is then used normally.
The second period, occurring immediately after the block period,
is the font swap period.
During this period,
if the font face is not loaded,
any element attempting to use it must instead
render with a fallback font face.
If the font face successfully loads during the swap period,
the font face is then used normally.
The third period, occurring immediately after the swap period,
is the font failure period.
If the font face is not yet loaded when this period starts,
it’s marked as a failed load,
causing normal font fallback.
Otherwise, the font face is used normally.
To render with a fallback font face for a given element,
the user agent must find the first font face
specified in the element’s font-family list
which is already loaded,
and use that for rendering text.
Doing this must not trigger loads of any of the fallback fonts.
To render with an invisible fallback font face
for a given element,
find a font face as per "render with a fallback font face".
Create an anonymous font face
with the same metrics as the selected font face
but with all glyphs "invisible" (containing no "ink"),
and use that for rendering text.
Doing this must not trigger loads of any of the fallback fonts.
fallback and optional can result in some faces in a family being used
while others are required to fallback,
giving a "ransom note" look.
Perhaps require that all fonts in a family have the same behavior (all swapped in, or all fallback)?
See also the @font-feature-values for controlling the behavior on a font family basis.
The @font-face rule allows for linking to fonts
that are automatically fetched and activated when needed.
This allows authors to select a font
that closely matches the design goals for a given page
rather than limiting the font choice
to a set of fonts available on a given platform.
A set of font descriptors define the location of a font resource,
either locally or externally,
along with the style characteristics of an individual face.
Multiple @font-face rules can be used to construct font families with a variety of faces.
Using CSS font matching rules,
a user agent can selectively download
only those faces that are needed for a given piece of text.
The @font-face rule accepts the descriptors defined in this specification.
Each @font-facerule specifies a value for every font descriptor,
either implicitly or explicitly.
Those not given explicit values in the rule
take the initial value listed with each descriptor in this specification.
These descriptors apply solely within the context of the @font-face rule
in which they are defined,
and do not apply to document language elements.
There is no notion of which elements the descriptors apply to
or whether the values are inherited by child elements.
When a given descriptor occurs multiple times in a given @font-face rule,
only the last descriptor declaration is used
and all prior declarations for that descriptor are ignored.
To use a downloadable font called Gentium:
Gentium
p Gentium serif
The user agent will download Gentium and use it when rendering text
within paragraph elements. If for some reason the site serving the font
is unavailable, the default serif font will be used.
A given set of @font-face rules define a set of fonts
available for use within the documents that contain these rules.
When font matching is done, fonts defined using these rules
are considered before other available fonts on a system.
Downloaded fonts are only available to documents that reference them.
The process of activating these fonts must not
make them available to other applications
or to documents that don’t directly link to the same font.
User agent implementers might consider it convenient
to use downloaded fonts when rendering characters
in other documents for which
no other available font exists
as part of the installed font fallback procedure.
However, this would cause a security leak
since the contents of one page would be able to affect other pages,
something an attacker could use as an attack vector.
These restrictions do not affect caching behavior,
fonts are cached the same way other web resources are cached.
This at-rule follows the forward-compatible parsing rules of CSS.
Like properties in a declaration block,
declarations of any descriptors that are not supported by the user agent
must be ignored.
@font-face rules require a font-family
and src descriptor;
if either of these are missing,
the @font-face rule must not be considered
when performing the font matching algorithm.
In cases where user agents have limited platform resources
or implement the ability to disable downloadable font resources,
@font-face rules must simply be ignored;
the behavior of individual descriptors
as defined in this specification
should not be altered.
This descriptor defines the font family name
that will be used in all CSS font family name matching.
It overrides the font family names
contained in the underlying font data.
If the font family name is the same
as a font family available in a given user’s environment,
it effectively hides the underlying font
for documents that use the stylesheet.
This permits a web author to freely choose font-family names
without worrying about conflicts with font family names
present in a given user’s environment.
Likewise, platform substitutions
for a given font family name
must not be used.
This descriptor specifies the resource containing font data.
Its value is
a prioritized,
comma-separated list
of external references
or locally-installed font face names.
When a font is needed
the user agent iterates over the set of references listed,
using the first one it can successfully parse and activate.
Parsing this descriptor is more complicated than parsing other descriptors;
see § 4.3.1 Parsing the src descriptor for the parsing rules.
Activation of a font involves downloading the file
or reading it from disk,
parsing it,
and perhaps additional user-agent-dependent steps.
Fonts containing invalid data
or local font faces
that are not found
are ignored
and the user agent loads the next font in the list.
While keywords are preferred to identify font formats,
for reasons of backwards compatibility the following strings are also accepted,
and have the same effect as if the equivalent modern syntax had been used.
String form
Equivalent syntax
format("woff2")
format(woff2)
format("woff")
format(woff)
format("truetype")
format(truetype)
format("opentype")
format(opentype)
format("collection")
format(collection)
format("woff2-variations")
format(woff2) tech(variations)
format("woff-variations")
format(woff) tech(variations)
format("truetype-variations")
format(truetype) tech(variations)
format("opentype-variations")
format(opentype) tech(variations)
Note: The CSS WG does not anticipate extending this list of format strings in the future.
If a component value is parsed correctly
and is of a font format or
font tech
that the UA supports,
add it to the list of supported sources.
If parsing a component value results in a parsing error
or its format or tech are unsupported,
do not add it to the list of supported sources.
If there are no supported entries at the end of this process,
the value for the src descriptor is a parse error.
These parsing rules allow for graceful fallback of fonts
for user agents which don’t support a particular font tech
or font format.
For example,
when incremental transfer is not supported,
a woff2 compressed version of the font
is supplied,
for optimal performance.
Then, for
incremental transfer,
the raw uncompressed OpenType font is provided,
portions of which will be loaded on demand.
opentype incremental
4.3.2. Loading an individual item in the src descriptor
As with other URLs in CSS,
the URL can be relative,
in which case
it is resolved relative to the location of the style sheet
containing the @font-face rule.
In the case of SVG fonts,
the URL points to an element within a document
containing SVG font definitions.
If the element reference is omitted,
a reference to the first defined font is implied.
Similarly, font container formats
that can contain more than one font
must load one and only one of the fonts
for a given @font-face rule.
Fragment identifiers are used to indicate which font to load;
these use the PostScript name of the font
as defined in [RFC8081].
Conformant user agents must not use a face
from the resource
unless its PostScript name exactly matches
the fragment identifier,
and must skip downloading the resource
if the given fragment identifier
cannot be a valid PostScript name.
External references consist of a URL,
followed by an optional hint
describing the format of the font resource referenced by that URL.
Conformant user agents
must skip downloading a font resource
if the format hint indicates an unsupported or unknown font format,
or if any of the font technologies are unsupported by the user agent.
If no format hint is supplied,
the user agent should download the font resource.
For example, the following shows how to load a WOFF 2 font if possible,
otherwise a WOFF 1,
otherwise use an OpenType font
bodytext
In this example, if woff2 is not supported,
an unsupported and fictitious "zebra" format is skipped
and the opentype font is downloaded:
Here an individual instance in a collection is loaded,
or if collections are not supported, download an individual font instead.
源ノ角ゴシック Code JP
4.3.3.1. Local font fallback
When authors would prefer to use
a locally available copy of a given font
and download it if it’s not,
can be used.
The locally-installed <font-family-name> argument to
is a format-specific string
that uniquely identifies a single font face
within a larger family.
The name can optionally be enclosed in quotes.
If unquoted,
the unquoted font family name processing conventions apply;
in other words,
the name must be a sequence of identifiers
separated by whitespace
which is converted to a string
by joining the identifiers together
separated by a single space;
and thus,
CSS-wide keywords such as inherit, and
<generic-font-family> keywords such as serif
are not allowed inside .
MyGentium
Gentium
For example, this use of would be an error:
foo
inherit
For OpenType and TrueType fonts,
this string is used to match only the Postscript name
or the full font name
in the name table of locally available fonts.
Which type of name is used varies by platform
and font,
so authors should include both of these names
to assure proper matching across platforms.
Platform substitutions for a given font name must not be used.
Just as an @font-face rule specifies
the characteristics of a single font
within a family,
the unique name used with
specifies a single font,
not an entire font family.
Defined in terms of OpenType font data,
the Postscript name is found in
the font’s
name table,
in the name record with nameID = 6
(see [OPENTYPE] for more details).
The Postscript name is the commonly used key for all fonts on OSX
and for Postscript CFF fonts under Windows.
The full font name (nameID = 4) is used as a unique key
for fonts with TrueType glyphs on Windows.
For OpenType fonts with multiple localizations of the full font name,
the US English version must be used
(language ID = 0x409 for Windows and language ID = 0 for Macintosh)
or the first localization
when a US English full font name is not available
(the OpenType specification recommends that
all fonts
minimally include US English names).
User agents that also match other full font names,
e.g. matching the Dutch name when the current system locale is set to Dutch,
are considered non-conformant.
Note: This is done,
not to prefer English,
but to avoid matching inconsistencies
across font versions and OS localizations,
since font style names (e.g. "Bold")
are frequently localized into many languages
and the set of localizations available
varies widely across platform and font version.
User agents that match a concatenation of
family name (nameID = 1) with
style name (nameID = 2)
are considered non-conformant.
Note: This also allows for referencing faces
that belong to larger families
that cannot otherwise be referenced.
Use a local font or reference an SVG font in another document:
Headline
Futura-Medium
Create an alias for local Japanese fonts on different platforms:
jpgothic
HiraKakuPro-W3 Meiryo IPAPGothic
Reference a font face that cannot be matched within a larger family:
Hoefler Text Ornaments
src: HoeflerText-Ornaments
Since localized fullnames never match,
a document with the header style rules below
would always render using the default serif font,
regardless whether a particular system locale parameter is set to Finnish or not:
A conformant user agent would never load the font 'gentium.eot'
in the example below,
since it is included in the first definition of the src descriptor
which is overridden by the second definition in the same @font-face rule:
These descriptors define the characteristics of a font face
and are used in the process of matching styles to specific faces.
For a font family defined with several @font-face rules,
user agents can either download all faces in the family
or use these descriptors to selectively download font faces that match actual styles used in document.
The meaning of the values for these descriptors
are the same as those for the corresponding font properties
except that relative keywords are not allowed,
bolder and lighter.
If these descriptors are omitted,
initial values are assumed.
If specified values are out of range
of the accepted values of the property of the same name,
the descriptor is treated as a parse error.
Ranges are accepted in these three descriptors in place of a single value.
Where a single value is specified,
it has the same meaning as a range with identical startpoint and endpoint.
User agents must swap the computed value of the startpoint and endpoint of the range
in order to forbid decreasing ranges.
Both endpoints are inclusive.
The ranges are used in the Font Matching Algorithm below.
The auto values for these three descriptors have the following effects:
For font selection purposes, the font is selected as if
the appropriate normal value
(normal, normal or normal)
is chosen
For variation axis clamping, clamping does not occur
The value for these font face style attributes
is used in place of the style implied by the underlying font data.
This allows authors to combine faces in flexible combinations,
even in situations where the original font data was arranged differently.
User agents that implement synthetic bolding and obliquing
must only apply synthetic styling in cases
where the font descriptors imply this is needed,
rather than based on the style attributes implied by the font data.
However, variation values applied to fonts defined with '@font-face'
will be clamped to both the values specified in these descriptors,
or implied by the application of variation parameters
(such as manipulation of the wght axis
to satisfy the requested font-weight),
as well as the values supported by the font file itself.
The font descriptors defined in this section
are used for selecting a font
from within the set of fonts defined by @font-face rules for a given family.
Consider a family containing a single, regular face:
BaskervilleSimple
Unstyled text would display using the regular face
defined in the @font-face rule:
However, italic text would display in most user agents
using synthetically obliqued glyphs from the regular face,
since a separate italic face is not defined:
Now consider a family for which an actual italic face is defined:
BaskervilleFull
BaskervilleFull
italic
The second @font-face rule
defines the font resource baskerville-italic.woff
to have style attributes of normal weight, normal stretch and italic style.
When displaying italic text,
the user agent will use this font,
since it’s the closest match for italic text.
Thus, the text will display using glyphs designed by a type designer
rather than using synthetically obliqued glyphs from the regular face:
The above @font-face rule indicates that lastima-varfont.woff should be used when
font-weight is between 100 and 399. Depending on if there are any other @font-face rules which specify
Lastima, lastima-varfont.woff might be used for values of font-weight
outside of the 100 - 399 range. For more details, see the § 5 Font Matching Algorithm.
The above @font-face rules indicate that lastima-varfont-lightrange.woff should be used when
font-weight is between 100 and 399, whereas lastima-varfont-heavyrange.woff should be used when
font-weight is between 400 and 700.
4.4.1.
Font width: the font-stretch legacy name alias
For historical reasons,
a font-stretch descriptor exists
which is a legacy name alias
and functions in the identical way to
the font-width descriptor.
This descriptor defines the set of Unicode codepoints that may be
supported by the font face for which it is declared. The descriptor
value is a comma-delimited list of Unicode range token (<unicode-range-token>)
values. The union of these ranges defines the set of codepoints that
serves as a hint for user agents when deciding whether or not to
download a font resource for a given text run.
Each <unicode-range-token> value is a
token made up of a "U+" or "u+" prefix
followed by a codepoint range in one of the three forms listed below.
Ranges that do not fit one of these forms are invalid
and cause the declaration to be ignored.
single codepoint (e.g. U+416)
a Unicode codepoint, represented as one to six hexadecimal digits
interval range (e.g. U+400-4ff)
represented as two hyphen-separated Unicode codepoints
indicating the inclusive start and end codepoints of a range
wildcard range (e.g. U+4??)
defined by the set of codepoints implied when
trailing '?' characters signify any hexadecimal digit
Individual codepoints are written using hexadecimal values that correspond to
Unicode character codepoints.
Unicode codepoint values must be between 0 and 10FFFF inclusive. Digit
values of codepoints are ASCII case-insensitive. For interval ranges,
the start and end codepoints must be within the range noted above and
the end codepoint must be greater than or equal to the start
codepoint.
Wildcard ranges specified with ‘?’ that lack an
initial digit (e.g. "U+???") are valid and equivalent
to a wildcard range with an initial zero digit (e.g. "U+0???" = "U+0000-0FFF").
Wildcard ranges that extend beyond the range of
Unicode codepoints are invalid. Because of this, the maximum
number of trailing '?' wildcard characters is five, even though the
UNICODE-RANGE
token accepts six.
Within the comma-delimited list of Unicode ranges in a
unicode-range descriptor declaration, ranges may overlap. The union
of these ranges defines the set of codepoints for which the
corresponding font may be used. User agents must not download or use
the font for codepoints outside this set. User agents may normalize
the list of ranges into a list that is different but represents the
same set of codepoints.
The associated font might not contain glyphs for the entire set of
codepoints defined by the unicode-range descriptor. When the font
is used, the effective character map is the intersection of the
codepoints defined by unicode-range with the font’s character map.
This allows authors to define supported ranges in terms of broad
ranges without worrying about the precise codepoint ranges supported
by the underlying font.
4.5.1. Using character ranges to define composite fonts
Multiple @font-face rules with different unicode ranges for the same
family and style descriptor values can be used to create composite fonts
that mix the glyphs from different fonts for different scripts. This
can be used to combine fonts that only contain glyphs for a single
script (e.g. Latin, Greek, Cyrillic) or it can be used by authors as a
way of segmenting a font into fonts for commonly used characters and
less frequently used characters. Since the user agent will only pull
down the fonts it needs this helps reduce page bandwidth.
If the unicode ranges overlap for a set of @font-face rules with the
same family and style descriptor values, the rules are ordered in the
reverse order they were defined; the last rule defined is the first to
be checked for a given character.
Example ranges for specific languages or characters:
code range for Japanese kanji, hiragana and katakana characters plus yen/yuan symbol
The BBC provides news services in a wide variety of languages, many
that are not well supported across all platforms. Using an @font-face
rule, the BBC could provide a font for any of these languages, as it
already does via a manual font download.
BBCBengali
U U
Technical documents often require a wide range of symbols. The STIX
Fonts project is one project aimed at providing fonts to support a wide
range of technical typesetting in a standardized way. The example below
shows the use of a font that provides glyphs for many of the
mathematical and technical symbol ranges within Unicode:
STIXGeneral
STIXGeneral
U U U U
This example shows how an author can override the glyphs used for
Latin characters in a Japanese font with glyphs from a different font.
The first rule specifies no range so it defaults to the entire range.
The range specified in the second rule overlaps but takes precedence
because it is defined later.
JapaneseWithGentium
MSMincho
JapaneseWithGentium
U
Consider a family constructed to optimize bandwidth by separating out
Latin, Japanese and other characters into different font files:
DroidSans
DroidSans
U U+ff??
DroidSans
U U U
For simple Latin text, only the font for Latin characters is downloaded:
body DroidSans
<p>This is that</p>
In this case the user agent first checks the unicode-range for the
font containing Latin characters (DroidSans.woff). Since all the
characters above are in the range U+0-5FF, the user agent downloads the
font and renders the text with that font.
Next, consider text that makes use of an arrow character (⇨):
<p>This ⇨ that<p>
The user agent again first checks the unicode-range of the font
containing Latin characters. Since U+2000-2300 includes the arrow
code point (U+21E8), the user agent downloads the font. For this
character however the Latin font does not have a matching glyph, so the
effective unicode-range used for font matching excludes this code point.
Next, the user agent evaluates the Japanese font. The unicode-range for
the Japanese font, U+3000-9FFF and U+ff??, does not include U+21E8, so
the user agent does not download the Japanese font.
Next the fallback font is considered. The @font-face rule for the
fallback font does not define unicode-range so its value defaults to
the range of all Unicode code points. The fallback font is downloaded and
used to render the arrow character.
These descriptors define initial settings that apply when the font defined by
an @font-face rule is rendered. They do not affect font selection.
Values are identical to those defined for the corresponding
font-feature-settings and
font-variation-settings properties defined below
except that the CSS-wide keywords are omitted. When multiple font
feature descriptors, properties, or variations are used, optionally along with a named instance,
the cumulative effect on
text rendering is detailed in the section
§ 7 Font Feature and Variation Resolution below.
These descriptors set features and variation values
on the font object which the @font-face rule represents,
rather than on an entire element.
Therefore, when using these descriptors,
only some glyphs in an element may be rendered with that feature,
due to § 5.3 Cluster matching.
4.7. Using named instances from variable fonts: the font-named-instance descriptor
If the font-named-instance descriptor is set
to a value other than auto,
then the appropriate stage in the § 7 Font Feature and Variation Resolution
will inspect the font file to find the first named instance in
the font which has a localized name equal to the given <string>
according to the rules given in § 5.1 Localized name matching.
If no such named instance exists,
this descriptor is treated as if it has a value of 'font-named-instance/auto'.
Otherwise, this named instance’s variation axis values
are applied to this position in the § 7 Font Feature and Variation Resolution.
For example, the following @font-face block will
apply the instance named "Grotesque"
but will override the "XHGT" axis to have a value of 0.7.
Note: Because the variation axis values supplied in the
font-weight, font-width, and font-style properties
are applied before
the value in the font-named-instance descriptor,
there is no need to change the value of those properties
when a named instance is desired.
4.8. Font request guidelines
4.8.1. Font loading guidelines
The @font-face rule is designed to allow lazy loading
of font resources that are only downloaded when used within a
document. A stylesheet can include @font-face rules for
a library of fonts of which only a select set are used; user agents
must only download those fonts that are referred to within the style
rules applicable to a given page. User agents that download all fonts
defined in @font-face rules without considering whether
those fonts are in fact used within a page are considered
non-conformant. In cases where a font might be downloaded in character
fallback cases, user agents may download a font if it’s contained within
the computed value of font-family
for a given text run.
GeometricModern
p
font-family: GeometricModern sans-serif
h2
font-family: Futura GeometricModern sans-serif
In cases where textual content is loaded before downloadable fonts are available,
user agents must render text according to the font-display descriptor of that @font-face block.
In cases where the font download fails, user agents must
display the text visibly. Authors are advised to use fallback fonts in
their font lists that closely match the metrics of the
downloadable fonts to avoid large page reflows where possible.
4.8.2.
Font fetching requirements
To fetch a font given a selected <url>url for @font-facerule,
fetchurl,
with ruleOrDeclaration being rule,
destination "font",
CORS mode "cors",
and processResponse being the following steps given responseres and null, failure or a
byte stream stream:
If stream is null, return.
Load a font from stream according to its type.
Note: The implications of this for authors are that fonts
will typically not be loaded cross-origin unless authors specifically
take steps to permit cross-origin loads. Sites can explicitly allow
cross-site loading of font data using the Access-Control-Allow-Origin
HTTP header. For other schemes, no explicit mechanism to allow
cross-origin loading, beyond what is permitted by the
fetch
algorithm, is defined or required.
For the examples given below, assume that a document is located at
//example.com/page.html and all URLs link to valid
font resources supported by the user agent.
Fonts defined with the src descriptor values below will be loaded:
src:
src:
src:
Fonts defined with the src descriptor values below will fail to load:
src:
src:
4.9.
Controlling Font Display Per Font-Face: the font-display descriptor
The font-display descriptor for @font-face
determines how a font face is displayed,
based on whether and when it is downloaded and ready to use.
Note: For all of these values,
user agents may use slightly different durations,
or more sophisticated behaviors that can’t be directly expressed in the font-display syntax,
in order to provide more useful behavior for their users.
They may also provide the ability for users to override author-chosen behavior
with something more desirable;
for example, forcing all fonts to have a 0sblock period.
auto
The font display policy is user-agent-defined.
Note: Many browsers have a default policy similar to that specified by block.
block
Gives the font face a short block period
(3s is recommended in most cases)
and an infinite swap period.
Note: In other words, the browser draws "invisible" text at first if it’s not loaded,
but swaps the font face in as soon as it loads.
This value must only be used when rendering text in a particular font is required for the page to be usable.
It must only be used for small pieces of text.
For example, badly designed "icon fonts" might associate a "⎙" (printer) icon
with an unrelated character like "P",
so if the text is displayed with a fallback font instead
there will be confusing letters scattered around the page
rather than the desired icon.
In this case, temporary blank spots are better than using a fallback font.
(However, the fallback font is used eventually,
as having confusing letters scattered around the page
is better than having links and such never show up at all.)
Gives the font face an extremely small block period
(100ms or less is recommended in most cases)
and an infinite swap period.
Note: In other words, the browser draws the text immediately with a fallback if the font face isn’t loaded,
but swaps the font face in as soon as it loads.
This value should only be used when rendering text in a particular font is very important for the page,
but rendering in any font will still get a correct message across.
It should only be used for small pieces of text.
For example,
if a website has a custom font for rendering their logo,
rendering that logo correctly is fairly important for branding purposes,
but displaying the logo in any font will at least get the point across without confusion.
fallback
Gives the font face an extremely small block period
(100ms or less is recommended in most cases)
and a short swap period
(3s is recommended in most cases).
Note: In other words, the font face is rendered with a fallback at first if it’s not loaded,
but it’s swapped in as soon as it loads.
However, if too much time passes,
the fallback will be used for the rest of the page’s lifetime instead.
This value should be used for body text,
or any other text where the use of the chosen font is useful and desired,
but it’s acceptable for the user to see the text in a fallback font.
This value is appropriate to use for large pieces of text.
For example,
in large pieces of body text,
it’s most important just to get the text rendered quickly,
so the user can begin to read as quickly as possible.
Further, once the user has started reading,
they shouldn’t be disturbed by the text suddenly "shifting"
as a new font is swapped in,
as that’s distracting and annoying to re-find where one was in the text.
optional
If the font can be loaded "immediately"
(such that it’s available to be used for the "first paint" of the text),
the font is used.
Otherwise, the font is treated
as if its block period and swap period both expired before it finished loading.
If the font is not used due to this,
the user agent may choose to abort the font download,
or download it with a very low priority.
If the user agent believes it would be useful for the user,
it may avoid even starting the font download,
and proceed immediately to using a fallback font.
An optional font must never
cause the layout of the page to "jump" as it loads in.
A user agent may choose to slightly delay rendering an element using an optional font
to give it time to load from a possibly-slow local cache,
but once the text has been painted to the screen with a fallback font instead,
it must not be rendered with the optional font for the rest of the page’s lifetime.
This value should be used for body text,
or any other text where the chosen font is purely a decorative "nice-to-have".
It should be used anytime it is more important that the web page render quickly on first visit,
than it is that the user wait a longer time to see everything perfect immediately.
For example, body text is perfectly readable in one of the browser default fonts,
though a downloadable font face might be more attractive
and mesh with the site’s aesthetics better.
First time visitors to a site generally care far more about the site being quickly usable
than they do about the finer points of its display,
and optional provides a good behavior for them.
If they return later,
the desired font faces might have finished downloading,
giving them the "intended" experience without slowing down
either their first or subsequent visits.
Users on very slow connections might not ever receive the "intended" experience,
but optional ensures they can actually use the site,
rather than quitting and going elsewhere because the site takes too long to load.
To increase the chance that an optional font will be available
in time to use for rendering some text,
it is recommended that the user agent employ heuristics
to make it faster to access
or to judge when it would be worthwhile to delay rendering some text,
such as checking if the font is preloaded in HTML,
or moving the file from slower to faster in-memory caches
as soon as it is seen in a stylesheet
(before it’s known whether it will be used on the page).
These heuristics cannot be relied upon by authors, however;
it must be understood that the optional value
can result in the font never being used.
If a somewhat higher assurance of the font being used is needed,
authors should consider using the fallback value.
The font-display descriptor for @font-feature-values
determines how a font family is displayed,
by setting the "default" font-display value
for @font-face rules targeting the same font family.
When font-display is omitted in an @font-face rule,
the user agent uses the font-display value
set via @font-feature-values for the relevant font-family if one is set,
and otherwise defaults to auto.
This mechanism can be used to set a default display policy for an entire font-family,
and enables developers to set a display policy for @font-face rules
that are not directly under their control.
For example, when a font is served by a third-party font foundry,
the developer does not control the @font-face rules
but is still able to set a default font-display policy for the provided font-family.
The ability to set a default policy for an entire font-family
is also useful to avoid the ransom note effect
(i.e. mismatched font faces)
because the display policy is then applied to the entire font family.
This descriptor defines initial settings that apply when the font defined by an @font-face rule is rendered. It does not affect font selection. Values are identical to those defined for the font-language-override property defined below except that the value inherit is omitted. When multiple font feature descriptors, properties, or variations are used, the cumulative effect on text rendering is detailed in the section § 7 Font Feature and Variation Resolution below.
When the descriptor value is normal, the corresponding metric value is obtained from the
font file directly.
Note: User agents may draw data from different places from the font file as the metric values,
which results in different text layouts.
When the descriptor value is a percentage, the corresponding metric value is resolved as the
given percentage multiplied by the used font size. Negative values are invalid at parse time.
The percentage is resolved against different font sizes for different elements.
The outer span uses an ascent value of
10px, whereas the inner span uses 15px.
We may override the metrics of a local fallback font to match the primary font, which
is a web font. This reduces layout shifting when switching from fallback to the
primary font.
cool-web-font
fallback-to-local
Some Local Font
ascent-override:
<div style=>Title goes here</div>
<img src= alt=>
The image will not be vertically shifted when the user agent finishes loading and
switches to use the web font.
5. Font Matching Algorithm
The algorithm below describes how fonts are associated with individual runs of text.
For each character in the run
a font family is chosen
and a particular font face is selected
containing a glyph for that character.
The set of installed fonts available
to the Font Matching Algorithm
is explicitly undefined.
The default set of installed fonts will vary
by UA, platform, and locale;
it is important that users be able to customise
which installed fonts are available for rendering web pages
and to which generic font families, if any,
these fonts are mapped.
UAs may choose to expose all installed fonts to the web,
regardless of how that font was installed;
doing so is likely to have good internationalization properties
for users whose primary language is not supported
by fonts shipping with their operating system.
UAs may alternatively choose
to not initially expose any user-installed fonts to aid privacy on the web;
for the set of installed fonts a user has installed
is often used as a tracking vector to track users across the web.
Users should then be able to add to or remove fonts from that set,
according to their needs.
UAs may choose a hybrid approach, where some user-installed fonts
are initially exposed for internationalization,
but others aren’t.
Again, users should be able to customise this starting set.
UAs are expected to make informed decisions
on which fonts they expose to the web by default,
so as to balance between internationalization and privacy.
UAs are expected to also provide a convenient means
for users to add and subtract fonts
to meet their particular needs.
5.1. Localized name matching
Some font file formats allow
font faces to carry multiple localizations
of a particular string
(e.g. family name or named instance).
User agents must recognize and correctly match all of these names
independent of the underlying platform localization,
system API used, or document encoding.
For example,
for each of the fonts listed below,
the author can use either the Latin name
or the localized name
in the font-family property,
and the results will be identical on all systems:
👁 examples of localized family names Localized family names
User agents must match these names case insensitively,
using the "Default Caseless Matching" algorithm outlined in the Unicode specification [UNICODE].
This algorithm is detailed in section 3.13 entitled "Default Case Algorithms".
Specifically, the algorithm must be applied
without normalizing the strings involved
and without applying any language-specific tailorings.
The case folding method specified by this algorithm
uses the case mappings with status field "C" or "F"
in the CaseFolding.txt file of the Unicode Character Database.
Note: For authors this means that font family names are matched case insensitively,
whether those names exist in a platform font
or in the @font-face rules contained in a stylesheet.
Authors should take care to ensure that names
use a character sequence consistent with the actual font family name,
particularly when using combining characters such as diacritical marks.
For example, a family name that contains a lowercase a (U+0061)
followed by a combining ring (U+030A)
will not match a name that looks identical
but which uses the precomposed lowercase a-ring character (U+00E5)
instead of the combining sequence.
Note: Implementors should take care to verify that a given caseless string comparison implementation
uses this precise algorithm
and not assume that a given platform string matching routine follows it,
as many of these have locale-specific behavior
or use some level of string normalization.
5.2. Matching font styles
The procedure for choosing a font
for a given character in a run of text
consists of iterating over the font families named by the font-family property,
selecting a font face with the appropriate style
based on other font properties
and then determining whether a glyph exists for the given character.
This is done using the character map of the font,
data which maps characters to the default glyph for that character.
A font is considered to support a given character if
(1) the character is contained in the font’s character map and
(2) if required by the containing script, shaping information is available for that character.
Some legacy fonts might include a given character in the character map
but lack the shaping information
(e.g. OpenType layout tables
or Graphite tables)
necessary for correctly rendering text runs containing that character.
Codepoint sequences consisting of a base character
followed by a sequence of combining characters
are treated slightly differently,
see the section on cluster matching below.
For this procedure,
the default font face for a given font family
is defined to be the face that would be selected
if all font style properties were set to their initial value.
Using the computed font property values for a given element,
the user agent starts with the first family name
specified by the font-family property.
If the family name is a generic family keyword, the user agent
looks up the appropriate font family name to be used. User
agents may choose the generic font family to use based on the
language of the containing element or the Unicode range of the
character.
For other family names, the user agent attempts to find the
family name among fonts defined via @font-face rules and then
among available installed fonts
(this may include font aliases),
matching names with a
§ 5.1 Localized name matching as outlined
in the section above.
If the font
resources defined for a given face in an @font-face rule are either
not available or contain invalid font data, then the face should be
treated as not present in the family. If no faces are present for a
family defined via @font-face rules, the family should be treated as
missing; matching a platform font with the same name must not occur
in this case.
If a font family match occurs, the user agent assembles the set
of font faces in that family and then narrows the set to a single
face using other font properties in the order given below. Fonts might be
present in this group which can support a range of
font-width, font-style, or font-weight properties. In
this case, the algorithm proceeds as if each supported combination of
values is a unique font in the set. If such a font is ultimately
selected by this algorithm, particular values for
font-width, font-style, and font-weight must be applied
before any layout or rendering occurs. The application of these values
must be applied in the Apply
font matching variations step detailed in
§ 7 Font Feature and Variation Resolution.
A group
of faces defined via @font-face rules with identical font
descriptor values but differing unicode-range values are considered to be
a single composite font face for this step:
font-width is tried first. If a font
does not have any concept of varying strengths of width values, its width value
is mapped according table in the property definition.
If the matching set includes faces with width values
containing the font-width desired value, faces with width values which do not include the desired width value
are removed from the matching set. If there is no face
which contains the desired value, a width value is chosen using the rules below:
If the desired width value is less than or equal to 100%, width values below the
desired width value are checked in descending order followed by
width values above the desired width value in ascending order until a
match is found.
Otherwise, width values above the
desired width value are checked in ascending order followed by
width values below the desired width value in descending order until a
match is found.
Once the
closest matching width has been determined by this process,
faces with widths which do not include this determined width are removed from the matching set.
This search algorithm can be thought of as a distance function, where the lowest-distance value present in the font family is selected, and all fonts not including that value are eliminated.
Consider a font family with three fonts, named A, B, and C, each with associated supported ranges for the font-width descriptor. If an element is styled with "font-width: 125%", the search algorithm can be visualized as follows:
The font width ranges supported by fonts A, B, and C are shown in the graph above. As you can see, because font B contains the minimum width value across the entire family, font B would be selected by this algorithm. However, if font B were somehow eliminated from the family, font C would then contain the lowest distance in the family, so it would be selected.
Similar to the previous example, here is the conceptual distance graph for an element styled with "font-width: 75%":
As you can see, because font B contains the minimum width value across the entire family, font B would be selected by this algorithm. However, if font B were somehow eliminated from the family, font A would then contain the lowest distance in the family, so it would be selected.
font-style is tried next
(see § 5.2 Matching font styles).
If a font does not have any concept of varying strengths of italics or oblique angles, its style
is mapped according to the description in the font-style property definition.
If the matching set includes faces with italic values containing the mapped value of italic,
then faces with italic values which do not include the desired italic mapped value are removed from the matching set.
Otherwise, italic values above the desired italic value are checked in ascending order followed by
italic values below the desired italic value, until 0 is hit. Only positive values of italic values are checked
in this stage.
If no match is found, oblique values greater than or equal to 11deg are checked in ascending order
followed by oblique values below 11deg in descending order, until 0 is hit. Only positive values of oblique values
are checked in this stage.
As you can see, because font D contains the minimum italic value across the entire family, font D would be selected by this algorithm. However, if font D were somehow eliminated from the family, font E would then contain the lowest distance in the family, so it would be selected. If E were eliminated, C would be selected. If C were eliminated, font B would not be chosen immediately; instead, oblique values would be consulted and an oblique value might be chosen. However, if no oblique value is chosen, font B would then be selected, followed by font A.
If the value of font-style is oblique and the requested angle is greater than or equal to 11deg,
If the matching set includes faces with oblique values containing the value of oblique,
faces with oblique values which do not include the desired oblique value are removed from the matching set.
Otherwise, oblique values above the desired oblique value are checked in ascending order followed by
oblique values below the desired oblique value, until 0 is hit. Only positive values of oblique values are checked in this stage.
For variable fonts with a slnt axis,
a match is created by setting the slnt value
with the specified oblique value.
Otherwise, if font-synthesis-style has the value
auto,
then a fallback match is produced
by geometric shearing to the specified oblique value.
If no match is found, italic values greater than or equal to 1 are checked in ascending order
followed by italic values below 1 in descending order, until 0 is hit. Only positive values of italic values are checked in this stage.
If no match is found, oblique values less than or equal to 0deg are checked in descending order until a match is found.
If no match is found, italic values less than or equal to 0 are checked in descending order until a match is found.
Similar to the previous example, here is the conceptual distance graph for an element styled with "font-style: oblique 40deg":
As you can see, because font D contains the minimum oblique value across the entire family, font D would be selected by this algorithm. However, if font D were somehow eliminated from the family, font E would then contain the lowest distance in the family, so it would be selected. If E were eliminated, C would be selected. If C were eliminated, font B would not be chosen immediately; instead, italic values would be consulted and an italic value might be chosen. However, if no italic value is chosen, font B would then be selected, followed by font A.
If the value of font-style is oblique and the requested angle is greater than or equal to 0deg and less than 11deg,
If the matching set includes faces with oblique values containing the value of oblique,
faces with oblique values which do not include the desired oblique value are removed from the matching
set.
Otherwise, oblique values below the desired oblique value are checked in descending order until 0 is hit, followed by
oblique values above the desired oblique value. Only positive values of oblique values are checked
in this stage.
For variable fonts with a slnt axis,
a match is created by setting the slnt value
with the specified oblique value.
Otherwise, if font-synthesis-style has the value auto,
then a fallback match is produced
by geometric shearing to the specified oblique value.
If no match is found, italic values less than 1 are checked in descending order until 0 is hit,
followed by italic values above 1 in ascending order. Only positive values of italic values
are checked in this stage.
If no match is found, oblique values less than or equal to 0deg are checked in descending order until a match is found.
If no match is found, italic values less than or equal to 0 are checked in descending order until a match is found.
Similar to the previous example, here is the conceptual distance graph for an element styled with "font-style: oblique 13deg":
As you can see, because font D contains the minimum oblique value across the entire family, font D would be selected by this algorithm. However, if font D were somehow eliminated from the family, font C would then contain the lowest distance in the family, so it would be selected. If C were eliminated, E would be selected. If E were eliminated, font B would not be chosen immediately; instead, italic values would be consulted and an italic value might be chosen. However, if no italic value is chosen, font B would then be selected, followed by font A.
If the value of font-style is oblique and the requested angle is less than 0deg and greater than -11deg, follow the steps above, except with the negated values and opposite directions. If the value of font-style is oblique and the requested angle is less than or equal to -11deg, follow the steps above, except with the negated values and opposite directions.
As you can see, because font C contains the minimum oblique value across the entire family, font C would be selected by this algorithm. However, if font C were somehow eliminated from the family, font B would not be chosen immediately; instead, italic values would be consulted and an italic value might be chosen. However, if no italic value is chosen, font B would then be selected, followed by font A.
If an oblique angle was found in the above search, all faces which don’t include that oblique angle are excluded from the matching set. Otherwise, if an italic value was found in the above search, all faces which don’t include that italic value are excluded from the matching set.
User agents are not required to distinguish between italic and oblique fonts. In such user agents, the font-style matching steps above are performed by mapping both italic values and oblique angles onto a common scale. The exact nature of this mapping is undefined, however, an italic value of 1 must map to the same value that an oblique angle of 11deg maps to. Within font
families defined via @font-face rules, italic and oblique
faces must be distinguished using the value of the
font-style descriptor.
For families that lack any italic or oblique faces, user agents
may create artificial oblique faces, if this is permitted by the
value of the font-synthesis property.
font-weight is matched next. If a font does not have any concept of varying strengths of weights, its weight is mapped according list in the property definition. If bolder/lighter relative weights are used, the effective weight is calculated based on the inherited weight value, as described in the definition of the font-weight property. If the matching set after performing the steps above includes faces with weight values containing the font-weight desired value, faces with weight values which do not include the desired font-weight value are removed from the matching set. If there is no face which contains the desired value, a weight value is chosen using the rules below:
If the desired weight is inclusively between 400 and 500, weights greater than or equal to the target weight are checked in ascending order until 500 is hit and checked, followed by weights less than the target weight in descending order, followed by weights greater than 500, until a match is found.
If the desired weight is less than 400, weights less than or equal to the
desired weight are checked in descending order followed by
weights above the desired weight in ascending order until a
match is found.
If the desired weight is greater than 500, weights greater than or equal to the
desired weight are checked in ascending order followed by
weights below the desired weight in descending order until a
match is found.
Similar to the previous example, here is the conceptual distance graph for an element styled with "font-weight: 400":
As you can see, because font B contains the minimum distance across the entire family, font B would be selected by this algorithm. However, if font B were somehow eliminated from the family, font C would then contain the lowest distance in the family, so it would be selected. If C were eliminated, D would be selected, followed by fonts A and then E.
Similar to the previous example, here is the conceptual distance graph for an element styled with "font-weight: 450":
As you can see, because font C contains the minimum distance across the entire family, font C would be selected by this algorithm. However, if font C were somehow eliminated from the family, font D would then contain the lowest distance in the family, so it would be selected. If D were also eliminated, B would be selected, followed by fonts A and then E.
Similar to the previous example, here is the conceptual distance graph for an element styled with "font-weight: 500":
As you can see, because font D contains the minimum distance across the entire family, font D would be selected by this algorithm. However, if font D were somehow eliminated from the family, font B would then contain the lowest distance in the family, so it would be selected. If B were eliminated, C would be selected, followed by fonts B, A, and then E.
Similar to the previous example, here is the conceptual distance graph for an element styled with "font-weight: 300":
As you can see, because font B contains the minimum distance across the entire family, font B would be selected by this algorithm. However, if font B were somehow eliminated from the family, font A would then contain the lowest distance in the family, so it would be selected. If A were eliminated, C would be selected.
Once the closest matching weight has been determined by this process,
faces with weights which do not include this determined weight are removed from the matching set.
Note: There is a small behavior change between [CSS-FONTS-3] and this specification
with the animation of the font-weight property.
Previously, interpolated values of font-weight were rounded to their closest multiple of 100,
and the font-matching algorithm was run on these rounded values.
In this specification, the font-matching algorithm is able to accept any value,
so no rounding occurs.
The small behavior change is due to the discontinuous nature of the font-matching algorithm.
font-size must be
matched within a UA-dependent margin of tolerance. (Typically, sizes
for scalable fonts are rounded to the nearest whole pixel, while the
tolerance for bitmapped fonts could be as large as 20%.) Further
computations, e.g., by em values in other properties, are based on
the font-size value that
is used, not the one that is specified.
Note that more than one font might be remaining in the matching set after performing
the above steps. If so, the user agent must choose a single font from
the matching set and continue these steps with it. The choice of which
font to choose can differ between multiple user agents and multiple operating
system platforms; however, it must not differ between two elements in the same document.
If the matched face is defined via @font-face
rules, user agents must use the procedure below to select a single
font:
If the font resource has not been loaded and the range of
characters defined by the unicode-range descriptor value
includes the character in question, load the font.
After downloading, if the effective character map
supports the character in question, select that font.
When the matched face is a composite face, user agents must use
the procedure above on each of the faces in the composite face in
reverse order of @font-face rule definition.
While the download occurs, user agents must either wait until the
font is downloaded or render once with substituted font metrics and
render again once the font is downloaded.
If no matching face exists or the matched face does not contain
a glyph for the character to be rendered, the next family name is
selected and the previous three steps repeated. Glyphs from other
faces in the family are not considered. The only exception is that
user agents may optionally substitute a synthetically obliqued version of the
default face if that face supports a given glyph and synthesis
of these faces is permitted by the value of the font-synthesis property.
For example, a synthetic italic version of the regular face might be used if the
italic face doesn’t support glyphs for Arabic.
If there are no more font families to be evaluated and no matching
face has been found, then the user agent performs an installed font
fallback procedure to find the best match for the character to be
rendered. The result of this procedure can vary across user agents.
If a particular character cannot be displayed using any
font, the user agent should indicate by some means that a
character is not being displayed, displaying either a
symbolic representation of the missing glyph (e.g. using a Last
Resort Font) or using the missing character glyph from
a default font.
Optimizations of this process are allowed
provided that an implementation behaves as if the algorithm had been followed exactly.
Matching occurs in a well-defined order
to ensure that the results are as consistent as possible across user agents,
given an identical set of available fonts and rendering tech.
The first available font,
used for example in the definition of font-relative lengths such as ex
or in the definition of the line-height property,
is defined to be the first font
for which the character U+0020 (space)
is not excluded by a unicode-range,
given the font families in the font-family list
(or a user agent’s default font if none are available).
Installed fonts referenced directly by family name,
rather than via @font-face rules,
are considered to have a unicode-range
that covers the entire Unicode code space.
Note: it does not matter whether that font actually has a glyph for the space character.
5.3.
Cluster matching
When text contains characters such as combining marks,
ideally
the base character should be rendered
using the same font as the mark,
this assures proper placement of the mark.
For this reason,
the font matching algorithm for clusters
is more specialized
than the general case of matching a single character by itself.
For sequences containing variation selectors,
which indicate the precise glyph to be used for a given character,
user agents always attempt installed font fallback
to find the appropriate glyph
before using the default glyph of the base character.
A sequence of codepoints containing combining mark
or other modifiers
is termed a grapheme cluster
(see [CSS3TEXT] and [UAX29] for a more complete description).
For a given cluster containing a base character,
b and a sequence of combining characters
c1, c2…, the entire cluster is matched using these steps:
For each family in the font list,
a face is chosen using the style selection rules
defined in the previous section.
If all characters in the sequence b + c1 + c2 …
are completely supported by the font,
select this font for the sequence.
If a sequence of multiple codepoints
is canonically equivalent to a single character
and the font supports that character,
select this font for the sequence
and use the glyph associated with
the canonically equivalent character
for the entire cluster.
If no font was found in the font list in step 1:
If c1 is a variation selector,
system fallback must be used
to find a font that supports
the full sequence of b + c1.
If no font on the system supports the full sequence,
match the single character b
using the normal procedure for matching
single characters
and ignore the variation selector.
Note: a sequence with more than one variation selector
must be treated as an encoding error
and the trailing selectors must be ignored. [UNICODE]
Otherwise, the user agent may optionally use
installed font fallback
to match a font that supports
the entire cluster.
If no font is found in step 2,
use the matching sequence
from step 1 to determine the longest sequence
that is completely supported
by a font in the font list
and attempt to match the remaining combining characters
separately using the rules for single characters.
5.4.
Character handling issues
CSS font matching is always performed on
text runs containing Unicode characters [UNICODE],
so documents using legacy encodings are assumed
to have been transcoded before matching fonts.
For fonts containing character maps
for both legacy encodings and Unicode,
the contents of the legacy encoding character map
must have no effect on the results
of the font matching process.
The font matching process does not assume
that text runs are in either normalized or denormalized form
(see [CHARMOD-NORM] for more details).
Fonts may only support precomposed forms
and not the decomposed sequence of base character plus combining marks.
Authors should always tailor their choice of fonts to their content,
including whether that content contains
normalized or denormalized character streams.
If a given character is a Private-Use Area Unicode codepoint,
user agents must only
match font families named in the font-family list
that are not generic families.
If none of the families
named in the font-family list
contain a glyph for that codepoint,
user agents must display some form of missing glyph symbol
for that character
rather than attempting installed font fallback
for that codepoint.
When matching the replacement character U+FFFD,
user agents may skip the font matching process
and immediately display
some form of missing glyph symbol,
they are not required to display
the glyph from the font
that would be selected
by the font matching process.
In general,
the fonts for a given family
will all have the same
or similar character maps.
The process outlined here
is designed to handle even font families
containing faces with widely variant character maps.
However,
authors are cautioned that
the use of such families
can lead to unexpected results.
6.
Font Feature Properties
Modern font technologies support a variety of
advanced typographic and language-specific font features.
Using these features,
a single font can provide glyphs
for a wide range of
ligatures,
contextual and stylistic alternates,
tabular and old-style figures,
small capitals,
automatic fractions,
swashes,
and alternates specific to a given language.
To allow authors control over these font capabilities,
the font-variant property has been expanded.
It now functions as a shorthand
for a set of properties
that provide control over
stylistic font features.
6.1.
Glyph selection and positioning
This section is non-normative
Simple fonts used for displaying Latin text
use a very basic processing model.
Fonts contain a character map
which maps each character
to a glyph for that character.
Glyphs for subsequent characters
are simply placed one after the other along a run of text.
Modern font formats
such as OpenType
and AAT (Apple Advanced Typography)
use a richer processing model.
The glyph for a given character can be
chosen and positioned
not just based on the codepoint of the character itself,
but also on adjacent characters
as well as the language,
script,
and features enabled for the text.
Font features may be
required for specific scripts,
or recommended as enabled by default
or they might be stylistic features
meant to be used under author control.
The point at which font selection and positioning happens
in the overall order of text processing operations
(such as text transformation, text orientation and text alignment)
is described in CSS Text 3 § A Text Processing Order of Operations.
For a detailed description of glyph processing
for OpenType fonts,
see [WINDOWS-GLYPH-PROC].
Stylistic font features can be classified
into two broad categories:
ones that affect the harmonization of glyph shapes
with the surrounding context,
such as kerning and ligature features,
and ones such as the small-caps,
subscript/superscript and alternate features
that affect shape selection.
The subproperties of font-variant listed below
are used to control these stylistic font features.
They do not control features
that are required for displaying certain scripts,
such as the OpenType features used when displaying Arabic or Indic language text.
They affect glyph selection and positioning,
but do not affect font selection
as described in the font matching section
(except in cases required for compatibility with CSS 2.1).
To assure consistent behavior across user agents,
the equivalent OpenType property settings
are listed for individual properties
and are normative.
When using other font formats
these should be used as a guideline
to map CSS font feature property values
to specific font features.
6.2.
Language-specific display
OpenType also supports language-specific glyph selection and
positioning, so that text can be displayed correctly in cases where
the language dictates a specific display behavior. Many languages
share a common script, but the shape of certain letters can vary
across those languages. For example, certain Cyrillic letters have
different shapes in Russian text than in Bulgarian. In Latin text,
it’s common to render "fi" with an explicit fi-ligature that lacks a
dot on the "i". However, in languages such as Turkish which uses both
a dotted-i and a dotless-i, it’s important to not use this ligature or
use a specialized version that contains a dot over the "i".
The example below shows language-specific variations based on stylistic
traditions found in Spanish, Italian and French orthography:
If the content language of the element is known according to the
rules of the document language,
user agents are required to infer the OpenType language system from
the content language and use that when selecting and positioning
glyphs using an OpenType font.
If a writing system has been explicitly specified,
it must take precedence over the customary one implied by the content language.
For OpenType fonts, in some cases it may be necessary to explicitly
declare the OpenType language to be used, for example when displaying
text in a given language that uses the typographic conventions of
another language or when the font does not explicitly support a given
language but supports a language that shares common typographic
conventions. The font-language-override property is used for this
purpose.
Kerning is the contextual adjustment
of inter-glyph spacing.
This property controls metric kerning,
kerning that utilizes adjustment data
contained in the font.
auto
Specifies that kerning is applied at the discretion of the user agent
normal
Specifies that kerning is applied
none
Specifies that kerning is not applied
For fonts that do not include kerning data
this property will have no visible effect.
When rendering with OpenType fonts,
the [OPENTYPE] specification suggests that
kerning be enabled by default.
When kerning is enabled,
the relevant OpenType kerning features are enabled
(for horizontal typographic modes
and for sideways typesetting in vertical typographic modes,
the kern feature;
for upright typesetting in vertical typographic modes,
the vkrn feature.
User agents must also support fonts
that only support kerning
via data contained in a
kern font table,
as detailed in the OpenType specification.
If the letter-spacing property is defined,
kerning adjustments are considered part of the default spacing
and letter spacing adjustments are made
after kerning has been applied.
When set to auto,
user agents can determine
whether to apply kerning or not
based on a number of factors:
text size,
script, or
other factors that influence text processing speed.
Authors who want proper kerning
should use normal
to explicitly enable kerning.
Likewise,
some authors may prefer to disable kerning
in situations where performance
is more important than precise appearance.
However, in well-designed modern implementations
the use of kerning generally
does not have a large impact
on text rendering speed.
A value of normal
specifies that common default features are enabled,
as described in detail
in the next section.
For OpenType fonts,
common ligatures and contextual forms are on by default,
discretionary and historical ligatures are not.
Specifies that all types of ligatures and contextual forms
covered by this property
are explicitly disabled.
In situations where ligatures are not considered necessary,
this may
improve the speed of text rendering.
common-ligatures
Enables display of common ligatures
(OpenType features: liga, clig).
For OpenType fonts,
common ligatures are enabled by default.
Disables display of common ligatures
(OpenType features: liga, clig).
discretionary-ligatures
Enables display of discretionary ligatures
(OpenType feature: dlig).
Which ligatures are discretionary or optional is decided by the type designer,
so authors will need to refer to
the documentation of a given font
to understand which ligatures are considered discretionary.
Disables display of historical ligatures
(OpenType feature: hlig).
contextual
Enables display of contextual alternates
(OpenType feature: calt).
Although not strictly a ligature feature,
like ligatures this feature is commonly used
to harmonize the shapes of glyphs with the surrounding context.
For OpenType fonts,
this feature is on by default.
This property is used
to enable typographic subscript and superscript glyphs.
These are alternate glyphs
designed within the same em-box as default glyphs
and are intended to be laid out
on the same baseline
as the default glyphs,
with no resizing or repositioning of the baseline.
They are explicitly designed
to match the surrounding text
and to be more readable
without affecting the line height.
Subscript glyphs (top) vs. typical synthesized subscripts (bottom)
Individual values have the following meanings:
normal
None of the features listed below are enabled.
sub
Enables display of subscript variants
(OpenType feature: subs).
super
Enables display of superscript variants
(OpenType feature: sups).
Because of the semantic nature of subscripts and superscripts,
when the value is either sub or super
for a given contiguous run of text,
if a variant glyph is not available for all the characters in the run,
simulated glyphs should be synthesized
for all characters
using reduced forms of the glyphs
that would be used
without this feature applied.
This is done per run
to avoid a mixture of variant glyphs and synthesized ones
that would not align correctly.
In the case of OpenType fonts
that lack subscript or superscript glyphs
for a given character,
user agents must synthesize
appropriate subscript and superscript glyphs.
Superscript alternate glyph (left), synthesized superscript glyphs (middle), and incorrect mixture of the two (right)
In situations where text decorations
are only applied to runs of text
containing superscript or subscript glyphs,
the synthesized glyphs may be used,
to avoid problems with the placement of decorations.
In the past,
user agents have used font-size
and vertical-align
to simulate subscripts and superscripts
for the
sub
and sup elements.
To allow a backwards compatible way
of defining subscripts and superscripts,
it is recommended
that authors use conditional rules [CSS3-CONDITIONAL]
so that older user agents will still render
subscripts and superscripts via
the older mechanism.
Because smaller
is often used for these elements,
the effective scaling factor
applied to subscript and superscript text
varies depending upon the size.
For larger text,
the font size is often reduced by a third
but for smaller text sizes,
the reduction can be much less.
This allows subscripts and superscripts
to remain readable
even within elements using small text sizes.
User agents should consider this
when deciding how to synthesize
subscript and superscript glyphs.
The OpenType font format
defines subscript and superscript metrics
in the
OS/2 table[OPENTYPE]
but these are not always accurate in practice
and so cannot be relied upon
when synthesizing subscript and superscript glyphs.
Authors should note
that fonts typically only provide
subscript and superscript glyphs
for a subset of all characters
supported by the font.
For example,
while subscript and superscript glyphs
are often available for Latin numbers,
glyphs for punctuation and letter characters
are less frequently provided.
The synthetic fallback rules
defined for this property
try to ensure that subscripts and superscripts
will always appear
but the appearance may not match author expectations
if the font used does not provide
the appropriate alternate glyph
for all characters contained in a
subscript or superscript.
This property is not cumulative.
Applying it to elements
within a subscript or superscript
won’t nest the placement
of a subscript or superscript glyph.
Images contained within text runs
where the value of this property
is sub or super will be drawn
just as they would
if the value was normal.
Because of these limitations,
font-variant-position
is not recommended for use in user agent stylesheets.
Authors should use it
in cases where subscripts or superscripts
will only contain the narrow range of characters
supported by the fonts specified.
The variant glyphs
use the same baseline
as the default glyphs would use.
There is no shift in the placement along the baseline,
so the use of variant glyphs doesn’t affect
the height of the inline box
or alter the height of the linebox.
This makes superscript and subscript variants
ideal for situations where it’s important
that leading remain constant,
such as in multi-column layout.
A typical user agent default style for the sub element:
sub
sub
smaller
normal
Using font-variant-position
to specify typographic subscripts
in a way that will still show subscripts
in older user agents:
font-variant-position: sub
sub
baseline
inherit
sub
User agents that support the font-variant-position property
will select a subscript variant glyph
and render this
without adjusting the baseline or font-size.
Older user agents will ignore
the font-variant-position property definition
and use the standard defaults for subscripts.
This property allows
the selection of alternate glyphs
used for small or petite capitals or for titling.
These glyphs are specifically designed
to blend well with the surrounding normal glyphs,
to maintain the weight and readability
which suffers when text is simply resized
to fit this purpose.
Individual values have the following meanings:
normal
None of the features listed below are enabled.
small-caps
Enables display of small capitals
(OpenType feature: smcp).
Small-caps glyphs typically use the form of uppercase letters
but are reduced to the size of lowercase letters.
Enables display of small capitals for both upper and lowercase letters
(OpenType features: c2sc, smcp).
petite-caps
Enables display of petite capitals
(OpenType feature: pcap).
all-petite-caps
Enables display of petite capitals for both upper and lowercase letters
(OpenType features: c2pc, pcap).
unicase
Enables display of mixture of small capitals for uppercase letters
with normal lowercase letters
(OpenType feature: unic).
titling-caps
Enables display of titling capitals
(OpenType feature: titl).
Uppercase letter glyphs are often designed for use with lowercase letters.
When used in all uppercase titling sequences
they can appear too strong.
Titling capitals are designed specifically for this situation.
The availability of these glyphs
is based on whether a given feature is defined or not
in the feature list of the font.
User agents can optionally decide this
on a per-script basis
but should explicitly not decide this
on a per-character basis.
Some fonts may only support a subset
or none
of the features
described for this property.
For backwards compatibility with CSS 2.1,
if small-caps or all-small-caps is specified
but small-caps glyphs are not available for a given font,
user agents should simulate a small-caps font,
for example by taking a normal font
and replacing the glyphs for lowercase letters
with scaled versions of the glyphs
for uppercase characters
(replacing the glyphs for both upper and lowercase letters
in the case of all-small-caps).
The font-feature-settings property
also affects the decision of
whether or not to use a simulated small-caps font
(unlike CSS Fonts 3).
#example1 small-caps
#example2 small-caps
For fonts which don’t support small caps, both #example1 and #example2
should be rendered with synthesized small caps. However, for fonts which
do support small caps, #example1 should be rendered with native small
caps, while #example2 should be rendered without any small-caps (native
or synthesized).
To match the surrounding text,
a font may provide alternate glyphs
for caseless characters
when these features are enabled
but when a user agent simulates small capitals,
it must not attempt to simulate alternates
for codepoints which are considered caseless.
Caseless characters with small-caps, all-small-caps enabled
If either petite-caps or all-petite-caps' is specified
for a font that doesn’t support these features,
the property behaves as if
small-caps or all-small-caps, respectively,
had been specified.
If unicase is specified
for a font that doesn’t support that feature,
the property behaves as if small-caps was applied
only to lowercased uppercase letters.
If titling-caps is specified
with a font that does not support this feature,
this property has no visible effect.
When simulated small capital glyphs are used,
for scripts that lack uppercase and lowercase letters,
small-caps, all-small-caps, petite-caps,
all-petite-caps and unicase
have no visible effect.
When casing transforms are used
to simulate small capitals,
the casing transformations must match
those used for the text-transform property.
As a last resort,
unscaled uppercase letter glyphs
in a normal font
may replace glyphs in a small-caps font
so that the text appears
in all uppercase letters.
Specifies control over numerical forms. The example below shows how
some of these values can be combined to influence the rendering of
tabular data with fonts that support these features. Within normal
paragraph text, proportional numbers are used while tabular numbers
are used so that columns of numbers line up properly:
In the case of ordinal, although ordinal forms
are often the same as superscript forms, they are marked up
differently.
For superscripts, the variant property is only applied
to the sub-element containing the superscript:
sup super
x<sup></sup>
For ordinals, the variant property is applied
to the entire ordinal number rather than
just to the suffix (or to the containing paragraph):
.ordinal ordinal
<span class=></span>
In this case only the "th" will appear in ordinal form,
the digits will remain unchanged.
Depending upon the typographic traditions used in a given language,
ordinal forms may differ from superscript forms.
In Italian, for example,
ordinal forms sometimes include an underline in the ordinal design.
A simple flank steak marinade recipe,
rendered with automatic fractions and old-style numerals:
.amount oldstyle-nums diagonal-fractions
<h4>Steak marinade:</h4>
<ul>
<li><span class=></span> tbsp olive oil</li>
<li><span class=></span> tbsp lemon juice</li>
<li><span class=></span> tbsp soy sauce</li>
<li><span class=> /</span> tbsp dry minced onion</li>
<li><span class=> /</span> tsp italian seasoning</li>
<li>Salt & pepper</li>
</ul>
<p>Mix the meat with the marinade
and let it sit covered in the refrigerator
for a few hours or overnight.</p>
Note that the fraction feature
is only applied to values
not the entire paragraph.
Fonts often implement this feature using contextual rules
based on the use of the slash ('/') character.
As such, it’s not suitable for use as a paragraph-level style.
For any given character, fonts can provide a variety of alternate
glyphs in addition to the default glyph for that character. This
property provides control over the selection of these alternate
glyphs.
For many of the property values listed below, several different
alternate glyphs are available. How many alternates are available
and what they represent is font-specific, so these are each marked
font specific in the value definitions below. Because the nature
of these alternates is font-specific, the
@font-feature-values rule is used to define values for a
specific font family or set of families that associate a font-specific
numeric <font-feature-index> with a custom
<font-feature-value-name>, which is then used in this
property to select specific alternates:
Noble Script
p
Noble Script
flowing
When a particular <font-feature-value-name> has not
been defined for a given family or for a particular feature type, the
computed value must be the same as if it had been defined. However,
property values that contain these undefined <font-feature-value-name>
identifiers must be ignored when choosing glyphs.
p unknown-value
p normal
This allows values to be defined and used for a given set of font
families but ignored if fallback occurs, since the font family name
would be different. If a given value is outside the range supported by
a given font, the value is ignored. These values never apply to
generic font families.
Individual values have the following meanings:
normal
None of the features listed below are enabled.
historical-forms
Enables display of historical forms (OpenType feature: hist).
Enables replacement of default glyphs with ornaments, if provided in the font (font specific, OpenType feature: ornm <font-feature-index>).
Some fonts may offer ornament glyphs as alternates for a wide collection of characters; however, displaying arbitrary
characters (e.g., alphanumerics) as ornaments is poor practice as it distorts the semantics of the data. Font designers
are encouraged to encode all ornaments (except those explicitly encoded in the Unicode Dingbats blocks, etc.) as
alternates for the bullet character (U+2022) to allow authors to select the desired glyph using ornaments().
6.9.
Defining font specific alternates: the @font-feature-values rule
Several of the possible values of font-variant-alternates listed above
are labeled as font specific;
a font can define not just a single glyph for the feature,
but multiple possible glyph variants,
and associate each one with a numeric index
to let you choose which to turn on.
These numeric indexes are idiosyncratic to each face;
a swsh feature on one font face might turn on the swash version of capital Q,
while on another font face it turns on the swash version of the &.
Thus, specifying the index in font-feature-settings
requires that the author know exactly which font will be used on an element;
if they get it wrong (due to font fallback selecting a different font)
they might end up turning on an entirely different,
and undesirable,
feature to what they wanted!
It also means that the author can’t easily turn on similar features for elements with different fonts;
they have to individually set different font-feature-settings values for each
that uses the correct numeric indexes for the desired features.
To fix this issue,
the @font-feature-values rule lets an author assign,
for each font face,
a human-friendly name to specific feature indexes.
Using these friendly names,
an author can easily turn on similar features regardless of the font in use
(if they’ve defined that name for all the fonts),
and be sure they’re not accidentally turning on unrelated features
(as fonts without those names defined for them simply won’t do anything).
Using a commonly named value allows authors to use a single style rule
to cover a set of fonts for which the underlying selector is different for
each font. If either font in the example below is found, a circled number
glyph will be used:
Trying to turn on the "circled" forms for either font explicitly,
using @font-feature-values,
would require the author know for certain which font will be used;
if they expected "Otaru Kisa" and wrote font-feature-settings: "nalt" 1;,
it would turn on "circled" characters in Otara Kisa,
but would instead turn on boxed characters
if the system fell back to Taisho Gothic,
as that’s what Taisho Gothic associates with nalt !
6.9.1. Basic syntax
An @font-feature-values rule’s prelude
contains a list of font family names,
followed by a block containing multiple feature value blocks,
a special type of subsidiary at-rule.
Each feature value block contains declarations
mapping author-chosen human-friendly names
(such as "flowing")
to feature indexes for the associated feature.
The @font-feature-values prelude
is a comma-delimited list of font family names that match the definition of <font-family-name>
for the font-family property.
This means that only named font families are allowed;
rules that include generic or system fonts in the list of font families are syntax errors.
However, if a user agent defines a generic font to be a specific named font (e.g. Helvetica),
the settings associated with that family name will be used.
If syntax errors occur within the <font-family-name> list,
the entire rule @font-feature-values rule is invalid
and must be ignored.
Specifying the same <font-feature-value-type> more than once is valid;
their contents are cascaded together.
Each feature value block accepts a list of declarations,
the font feature value declarations,
where the declaration’s name can be any css identifier,
and the value must be a list of one or more non-negative <integer>s.
Each declaration’s value in @character-variant
must match the grammar <font-feature-index [0,99]><font-feature-index [0,∞]>,
or else the declaration is invalid and must be ignored.
Each declaration’s value in @styleset
must match the grammar <font-feature-index [0,20]>+,
or else the declaration is invalid and must be ignored.
Note: Each feature name is unique only within a single feature value block.
Between different feature value blocks,
or the same type of feature value blocks in separate @font-feature-values rules,
names can be reused without colliding.
For each <font-family-name> in the @font-feature-values prelude,
each font feature value declaration defines a mapping between a
(family name, feature block name, declaration name) tuple
and the list of one or more integers from the declaration’s value.
If the same tuple appears more than once in a document
(such as if a single block),
the last-defined one is used.
For example, the following all define the exact same set of font feature values:
foo
foo
foo
foo
foo
A syntax error within a font feature value declaration
makes the declaration invalid and ignored,
but does not invalidate the font feature value block it occurs in.
An unknown at-rule within a @font-feature-values block
(not using one of the predefined list of allowed at-keywords)
makes that at-rule invalid and ignored,
but does not invalidate the @font-feature-values rule.
Rules that are equivalent given syntax error handling:
Bongo
annotation
redrum
The example above is equivalent to:
Bongo
If multiple @font-feature-values rules are defined for
a given family, the resulting values definitions are the union of the
definitions contained within these rules. This allows a set of named
values to be defined for a given font family globally for a site and
specific additions made per-page.
Using both site-wide and per-page feature values:
site.css:
Mercury Serif
stacked-a:
page.css:
Mercury Serif
body
Mercury Serif serif
font-variant-alternates: stacked-g geometric-m
The character-variant() property value and the @character-variant descriptor allow two values,
which enables a feature (from the first value) and sets it to a given (second) value.
MM Greek
For the styleset() property value and @styleset rule,
multiple values indicate the style
sets to be enabled. Values between 1 and 99 enable OpenType features
ss01 through ss99.
However, the OpenType standard only officially defines
ss01 through ss20.
For OpenType fonts, values greater than 99 or equal to 0 do not
generate a syntax error when parsed but enable no OpenType features.
Mars Serif
curly-quotes:
code:
p.codeblock
font-variant-alternates: curly-quotes code
For <@character-variant>, a single value between 1 and 99 indicates
the enabling of OpenType feature cv01 through
cv99. For OpenType fonts, values greater than
99 or equal to 0 are ignored but do not generate a syntax error when parsed
but enable no OpenType features. When two values are listed, the first
value indicates the feature used and the second the value passed for
that feature. If more than two values are assigned to a given name, a
syntax error occurs and the entire
feature value definition is
ignored.
MM Greek
#title
font-variant-alternates: beta-3 gamma
p
font-variant-alternates: zeta zeta-2
.special
font-variant-alternates: zeta-2 zeta
Byzantine seal text displayed with character variants
In the figure above, the text in red is rendered using a font containing
character variants that mimic the character forms found on a Byzantine seal
from the 8th century A.D. Two lines below is the same text displayed in
a font without variants. Note the two variants for U and N used on the
seal.
Enables rendering of JIS83 forms
(OpenType feature: jp83).
jis90
Enables rendering of JIS90 forms
(OpenType feature: jp90).
jis04
Enables rendering of JIS2004 forms
(OpenType feature: jp04).
The various JIS variants reflect
the glyph forms defined in different Japanese national standards.
Fonts generally include glyphs
defined by the most recent national standard,
but it’s sometimes necessary
to use older variants,
to match signage for example.
simplified
Enables rendering of simplified forms
(OpenType feature: smpl).
traditional
Enables rendering of traditional forms
(OpenType feature: trad).
The simplified and traditional values
allow control over the glyph forms
for characters which have been simplified over time
but for which the older, traditional form
is still used in some contexts.
The exact set of characters and glyph forms
will vary to some degree
by the context for which a given font was designed.
Enables display of ruby variant glyphs
(OpenType feature: ruby).
Since ruby text is generally smaller
than the associated body text,
font designers can design special glyphs for use with ruby
that are more readable than scaled down versions
of the default glyphs.
Only glyph selection is affected,
there is no associated font scaling
or other change that affects line layout.
The red ruby text below
is shown with default glyphs (top)
and with ruby variant glyphs (bottom).
Note the slight difference
in stroke thickness.
The value normal
resets all subproperties of font-variant
to their initial value.
The none value
sets font-variant-ligatures to 'font-variant-ligatures/none'
and resets all other font feature properties to their initial value.
Like other shorthands,
using font-variant
resets unspecified font-variant subproperties
to their initial values.
This property provides
low-level control over OpenType font features.
It is intended as a way of providing access
to font features
that are not widely used
but are needed for a particular use case.
Authors should not use
font-feature-settings to set any of the font features in
the table below. Instead, please use the higher-level replacement properties, because:
The higher-level properties cascade individually. You can set one without setting the whole font-feature-settings list
Some higher-level properties can be synthesized for fonts that do not support the font feature.
For example, there is no font-variant value to control
Scientific Inferiors (small numerals used in chemical formulae).
Readability is enhanced by using them, so they must be enabled
using font-feature-settings:
The entire property value is set at once.
Unlike the font-variant properties,
there is no way to modify
the inherited value
by adding or removing individual features.
A value of
normal
means that no change
in glyph selection or positioning
occurs due to this property.
Feature tag values have the following syntax:
<feature-tag-value> = <opentype-tag> <integer ∞> | on | off ?
<opentype-tag> = <string>
The <opentype-tag> is a case-sensitive OpenType feature tag.
As specified in the OpenType specification [OPENTYPE],
feature tags contain four ASCII characters.
Tag strings longer or shorter than four characters,
or containing characters outside the U+20–7E codepoint range
are invalid.
Feature tags need only match
a feature tag defined in the font,
so they are not limited to
explicitly registered OpenType features.
Fonts defining custom feature tags
should follow the
tag name rules
defined in the OpenType specification [OPENTYPE-FEATURES].
Feature tags
not present in the font are ignored;
a user agent must not attempt to synthesize fallback behavior
based on these feature tags.
The one exception is that
user agents may synthetically support the kern feature
with fonts that contain kerning data in the form of a kern table
but lack kern feature support
in the GPOS table.
In general, authors should
use the font-kerning property
to explicitly enable or disable kerning
since this property always affects fonts
with either type of kerning data.
If present,
a value indicates an index used for glyph selection.
An <integer> value must be 0 or greater.
A value of 0 indicates that the feature is disabled.
For boolean features,
a value of 1 enables the feature.
For non-boolean features,
a value of 1 or greater enables the feature
and indicates the feature selection index. A
value of on is synonymous with 1
and off is synonymous with 0.
If the value is omitted, a value of 1 is assumed.
The computed value
of font-feature-settings is a map,
so any duplicates in the specified value
must not be preserved.
If the same feature tag appears more than once,
the value associated with the last appearance supersedes
any previous value for that axis.
The computed value contains the de-duplicated feature tags,
sorted in ascending order by code unit.
font-feature-settings: on
font-feature-settings:
font-feature-settings: off
font-feature-settings:
font-feature-settings:
font-feature-settings: off
font-feature-settings:
font-feature-settings: sinf
When values greater than the range
supported by the font
are specified,
the behavior is explicitly undefined.
For boolean features, in general these will enable the feature.
For non-boolean features,
out of range values will in general
be equivalent to a 0 value.
However,
in both cases the exact behavior
will depend upon the way the font is designed
(specifically, which type of lookup is used
to define the feature).
Implementations may choose to ignore
turning off features which
the OpenType specification says are always required,
such as required ligatures "rlig".
Although specifically defined for OpenType feature tags,
feature tags for other modern font formats
that support font features
may be added in the future.
Where possible,
features defined for other font formats
should attempt to follow
the pattern of registered OpenType tags.
The Japanese text below will be rendered with half-width kana characters:
Normally, authors can control the use of language-specific glyph substitutions and positioning
by setting the content language of an element, as described above:
<!-- Display text using S'gaw Karen specific features -->
<p lang=>...</p>
In some cases, authors may need to specify a language system
that differs from the content language, for example due to the need to mimic another language’s
typographic traditions. The font-language-override property allows authors to explicitly specify
the language system of the font, overriding the language system implied by the content language.
Values have the following meanings:
normal
specifies that when rendering with OpenType fonts,
the content language of the element is used to infer the OpenType language system
single four-character case-sensitive OpenType language system tag,
specifies the OpenType language system to be used instead of the language system implied by the language of the element.
If the string is shorter than four characters,
it is padded at the end with space (U+0020) characters
such that the length is 4, before being matched.
Unknown OpenType language system tags are silently ignored, and do not affect
glyph selection and placement.
The Universal Declaration of
Human Rights has been translated into a wide variety of languages. In Turkish,
Article 9 of this document might be marked up as below:
<body lang=>
<h4>Madde </h4>
<p>Hiç kimse keyfi olarak tutuklanamaz alıkonulanamaz veya sürülemez.</p>
Here the user agent uses the value of the lang attribute when rendering text and
appropriately renders this text without 'fi' ligatures. There is no need
to use the font-language-override property.
However, a given font may lack support for a specific language. In this
situation authors may need to use the typographic conventions of a related language
that are supported by that font:
<body lang=> <!-- Macedonian lang code -->
body
<h4>Члeн </h4>
<p>Никoj чoвeк нeмa дa бидe пoдлoжeн нa прoизвoлнo aпсeњe притвoр или прoгoнувaњe.</p>
As the content creator knows that font specified supports Serbian, the Macedonian text
here will be rendered using Serbian typographic conventions.
7.
Font Feature and Variation Resolution
As described in the previous section,
font features and variations can be enabled in a variety of ways,
either via the use of font-variant,
font-feature-settings, or
font-variation-settings in a style rule
or within an @font-face rule.
The resolution order for the union of these settings is defined below.
Features defined via CSS properties are applied on top of layout engine default features.
7.1.
Default features
For OpenType fonts,
user agents must enable the default features
defined in the OpenType documentation
for a given script and writing mode.
Required ligatures, common ligatures and contextual forms
must be enabled by default
(OpenType features: rlig, liga, clig, calt),
along with localized forms
(OpenType feature: locl),
and features required for proper display of composed characters and marks
(OpenType features: ccmp, mark, mkmk).
These features must always be enabled,
even when the value of the font-variant and font-feature-settings properties is normal.
Individual features are only disabled when explicitly overridden by the author,
as when font-variant-ligatures is set to 'font-variant-ligatures/no-common-ligatures'.
For handling complex scripts such as
Arabic,
Mongolian or
Devanagari
additional features are required.
For upright text within vertical text runs,
vertical alternates (OpenType feature: vert) must be enabled.
7.2.
Feature and variation precedence
General and font specific font feature property settings are
resolved in the order below, in ascending order of precedence. This ordering is
used to construct a combined list of font features that affect a given
text run.
Font features enabled by default are applied, including features required for a given script.
See § 7.1 Default features for a description of these.
The application of the value enabled by
font-style is affected by font selection, because this property might select an
italic or an oblique font. The value applied is the closest matching value as determined
by the font matching algorithm. User agents must apply
at most one value due to the font-style property; both "ital" and "slnt" values must
not be set together.
If the selected font is defined in an @font-face rule, then the values applied at this step
should be clamped to the value of the font-weight, font-width,
and font-style
descriptors in that @font-face rule.
Then, the values applied in this step should be clamped (possibly again) to the values
that are supported by the font.
The language specified by the inherited value of lang/xml:lang is applied.
If the font is defined via an @font-face rule, the font language override
implied by the font-language-override descriptor in the @font-face rule is applied.
If the font is defined via an @font-face rule, that @font-face rule includes
at least one valid font-named-instance descriptor
with a value other than 'font-named-instance/none',
and the loaded font resource includes a named instance with that name
according to the § 5.1 Localized name matching rules,
then all the variation values represented by that named instance are applied.
These values are clamped to the values that are supported by the font.
If the font is defined via an @font-face rule, the font variations
implied by the font-variation-settings descriptor in the @font-face rule are applied.
If the font is defined via an @font-face rule, the font features
implied by the font-feature-settings descriptor in the @font-face rule are applied.
The font language override implied by the value of the font-language-override property is applied.
Font variations implied by the value of the font-optical-sizing property are applied.
Font features implied by the value of the font-variant property,
the related font-variant subproperties and any other CSS property
that uses OpenType features (e.g. the font-kerning property) are applied.
Feature settings determined by properties other than font-variant or
font-feature-settings are applied. For example, setting a
non-default value for the letter-spacing property disables optional ligatures.
Font variations implied by the value of the font-variation-settings property are applied.
These values should be clamped to the values that are supported by the font.
This ordering allows authors to set up a general set of defaults
for fonts within their @font-face rules, then override them with
property settings for specific elements. General property settings
override the settings in @font-face rules and low-level font feature
settings override font-variant property settings.
For situations where the combined list of font feature settings
contains more than one value for the same feature, the last value is
used. When a font lacks support for a given underlying font feature,
text is simply rendered as if that font feature was not enabled; font
fallback does not occur and no attempt is made to synthesize the
feature except where explicitly defined for specific properties.
7.3.
Feature precedence examples
With the styles below, numbers are rendered proportionally when used within
a paragraph but are shown in tabular form within tables of prices:
body
proportional-nums
table.prices td
tabular-nums
The @font-face rule can also be used to access font features in locally available
fonts via the use of in the src descriptor of the @font-face definition:
BodyText
proportional-width
body BodyText serif
If available, a Japanese font "Hiragino Maru Gothic" will be used. When text
rendering occurs, Japanese kana will be proportionally spaced and Latin text will
be italicized. Text rendered with the fallback serif font will use default
rendering properties.
In the example below, discretionary ligatures are enabled only for a downloadable font
but are disabled within spans of class "special":
main
discretionary-ligatures
body main Helvetica
span.special no-discretionary-ligatures
Suppose one adds a rule using font-feature-settings to enable discretionary ligatures:
body main Helvetica
span
span.special no-discretionary-ligatures
In this case, discretionary ligatures will be rendered
within spans of class "special".
This is because both the font-feature-settings and font-variant-ligatures properties
apply to these spans.
Although the no-discretionary-ligatures setting of font-variant-ligatures
effectively disables the OpenType "dlig" feature,
because the font-feature-settings is resolved after that,
the "dlig" value reenables discretionary ligatures.
8.
Font Variation Properties
Note: The technology in use in this section is named "font variations."
An instance of one such font as a "variable font."
Typographically,
text rendered at different sizes
often benefits from slightly different visual representations.
For example, to aid reading at small text sizes,
strokes are often thicker with larger serifs.
Larger text often has a more delicate figure
with more contrast between thicker and thinner strokes.
auto
The user agent may modify the shape of glyphs
based on the font-size and the pixel density of the screen.
For OpenType and TrueType fonts using font variations,
this is often done by using the "opsz" font variation.
none
The user agent must not modify the shape of glyphs for optical size.
font-size must be considered when selecting a variation value for the "opsz" axis,
but other signals may also be considered.
Note: User agents are expected to supply a value for the "opsz" axis
which is close to the used value for font-size.
However, user agents might wish to consider other factors
such as pixel density of the screen,
or the solid angle the text subtends in the viewer’s retina.
Experiments have shown,
however, that disregarding these other ancillary factors
and using only font-size might be the best way for a user agent to select this value.
Pixel density as well as visual size of the text
may influence the variation value chosen for font-optical-sizing.
When either pixel density or visual size of the text
changes in response to a user operation or style change,
user agents must not choose a new value for this variation value
unless the change is layout-causing.
User agents are free to determine which changes are layout-causing.
Note: Some user operations,
such as pinch-zoom,
can be considered not-layout-causing
if they do not cause text to reflow.
However, other user operations,
such as increasing text size for accessibility purposes,
can be considered layout-causing because they cause text to reflow.
Similarly, the transform property can be considered not-layout-causing
because transforms generally do not cause text to reflow.
Each user-agent is free to decide
whether or not each operation is layout-changing or not.
User agents must not synthesize optical sizing
when it is not performed by the font directly.
User agents must not select a value for the "opsz" axis
which is not supported by the font used for rendering the text.
This can be accomplished by clamping a chosen value to the range supported by the font.
font-optical-sizing interacts with font-size-adjust. When applying optical sizing, User agents must
apply an optical sizing value appropriate for the adjusted font size, subject to the above restrictions.
This property provides low-level control
over OpenType or TrueType font variations.
It is intended as a way of providing access to font variations
that are not widely used but are needed for a particular use case.
Authors should not use
font-variation-settings to set any of the variation axes in
the table below. Instead, please use the higher-level replacement properties, because:
The higher-level properties cascade individually. You can set one without setting the whole font-variation-settings list
Some higher-level properties can be synthesized for fonts that do not support the font variation.
The use of font variation settings
does not affect font selection
where a fallback font is to be used,
where using font properties for the same axis
would have an effect on font selection.
When possible,
authors should generally use the other properties related to font variations
(such as font-optical-sizing),
and only use this property for special cases
where its use is the only way of accessing a particular infrequently used font variation.
A value of normal means that no change in glyph shape, matching, or positioning occurs due to this property.
The <opentype-tag> is a case-sensitive OpenType or TrueType variation axis name.
As specified in the OpenType / TrueType specifications,
axis names contain four ASCII characters.
Axis name strings longer or shorter than four characters,
or containing characters outside the U+20–7E codepoint range
are invalid.
Axis names need only match an axis tag defined in the font,
so they are not limited to explicitly registered OpenType / TrueType variation axes.
Fonts defining custom axis names
should follow the name rules defined in the OpenType specification.
If a given axis is not supported by a font,
its value setting is ignored and therefore has no effect;
a user agent must not attempt to synthesize fallback behavior based on these axis tags.
Other axis values within the same CSS font-variation-settings statement are not ignored.
If a variation axis is supported by a font
but its set value is greater or less than the predefined font range,
it will be clamped to the closest value supported by the font.
Values are allowed to be fractional or negative.
Note: it is preferable to obtain slanted faces
by using the font-style property.
However, if using the slnt axis directly,
via font-variation-settings,
remember that it is defined
with a positive angle meaning a counter-clockwise slant,
the opposite direction to CSS.
If the same axis name appears more than once, the value associated with the last appearance supersedes any previous value for that axis. This deduplication is observable by accessing the computed value of this property.
The computed value contains the de-duplicated axis names,
sorted in ascending order by code unit.
Although specifically defined for OpenType / TrueType variations,
variation axes for other modern font formats that support font variations
might be added in the future.
Where possible,
variations defined for other font formats
should attempt to follow the pattern of registered variation axes.
Animating font-variation-settings is possible
using the following mechanism. Two declarations of font-feature-settings can be animated between if they are "like".
"Like" declarations are ones where the same set of properties appear (in any order). Because successive duplicate properties are applied instead of prior duplicate properties, two declarations can be "like" even if they have differing number of properties.
If two declarations are "like"
then animation occurs pairwise between corresponding values in the declarations.
Otherwise, animation is not possible.
In this situation,
the "from" values of the animation are swapped to the "to" values
at an unspecified time during the animation.
9.
Color Font Support
Color fonts allow for font files to describe,
not just the contours describing the edges of glyphs,
but also the colors present inside the glyphs.
For some uses, such as emoji,
having the colors fixed in the font is appropriate.
For others, there is a need to control the colors used from a stylesheet.
CSS provides two ways to do this.
The font-palette property allows selecting
one of several different palettes
contained in the font.
The @font-palette-values rule allows overriding
one or more colors from within a palette
or even
the creation of an entirely different palette.
9.1.
Controlling Color Font Palettes: The font-palette property
Many color font file formats allow colors within glyphs to be parameterized.
In these fonts, colors are referenced by index when describing the geometry of each glyph.
These indices are resolved within a current active palette
using a lookup table present inside the font.
However, many fonts contain multiple palettes,
each containing a set of complementary colors
chosen by the font designer to provide pleasing visual results.
In addition, some color font file formats
provide a regular, uncolored glyph outline
as well as the colored versions.
specified keyword, identifier or <palette-mix()> function.
<palette-mix()> must be simplified to a single keyword or identifier if resulting palette is equivalent.
If the color-scheme property is set to a value other than normal',
user-agents should select the first light or dark palette as appropriate.
Otherwise, user-agents display the color font with the default palette
or default glyph colorisation.
User-agents should take the computed value of the color property into consideration
when a color font format requires use of the foreground color
In the COLR[OPENTYPE] table, color index 0xFFFF should be rendered according the color property.
For COLR/CPAL[OPENTYPE] fonts, font-palette: normal usually means rendering the font with the palette in the font at index 0.
light
Some color font formats include metadata
marking certain palettes as applicable on a light (close to white) background.
This keyword causes the user-agent
to use the first available palette in the font file marked this way.
If the font file format does not account for this metadata,
or no palette in the font is marked this way,
this value behaves as normal.
dark
Some color font formats include metadata
marking certain palettes as applicable on a dark (close to black) background.
This keyword causes the user-agent
to use the first available palette in the font file marked this way.
If the font file format does not account for this metadata,
or no palette in the font is marked this way,
this value behaves as normal.
This value identifies an CSS-defined palette to use.
Users can define a palette by using the @font-palette-values rule.
If no applicable @font-palette-values rule is present,
this value behaves as normal.
Note: Such a CSS-defined palette may
be a reference to an existing palette already present in the font,
be a modification of an existing palette already present in the font,
or be an entirely new palette.
The glyphs of the font must be rendered with a palette that is created by
interpolating for each palette color index between the matching color indices
of the first and second palette specified as arguments,
recursively resolving <palette-mix()> functions if needed.
For each palette color, the interpolation follows
the rules of the <color-mix()> function.
Note: The names 'font-palette/light' and 'font-palette/dark' describe the background the color palette is usable on,
not the colors in the color palette themselves.
When dark mode is in effect, use a dark mode palette
prefers-color-scheme: dark
.banner dark
Animate between palettes.
--pink
Nabla
--yellow
Nabla
animate-palette
from
--yellow
to
--pink
p
Nabla
animate-palette infinite alternate linear
The @font-palette-values rule defines a color palette
and associates that color palette with a specific font.
This allows a web author to select arbitrary <color>s to use inside a color font
rather than being limited to the preexisting palettes inside font files.
Note: The OpenType CPAL table is restricted to sRGB colors.
The <color>s in @font-palette-values allow any CSS color.
Some implementations currently use APIs which are restricted to sRGB
and may therefore map the specified colors to sRGB.
This is expected to be a temporary limitation.
Authors should not rely on this mapping to sRGB.
In addition, this rule’s association with a specific font
allows a name of a palette to apply differently to different fonts,
which allows similar colors to be used across multiple fonts
when multiple fonts are used in an element
(i.e. for font fallback).
Note: A web author might wish to create multiple palettes for a single font
in order to create multiple themes for their web content.
A web author might also wish to create matching palettes for multiple fonts
to achieve a consistent design across a collection of different fonts.
A @font-palette-values rule represents a palette of colors used in a font.
A palette consists of an ordered collection of colors.
Using the @font-palette-values allows a web author
to reference a palette existing within a font
as well as creating a palette populated with author-defined colors.
In addition, it allows overriding a set of colors from a palette in the font
with colors described by the web author.
A palette is always complete, meaning that it is impossible to describe
a palette which has missing colors. If colors would be missing, they are taken
from the palette within the font identified by the base-palette descriptor.
Math functions, such as calc(),
and also var(), and env(),
are valid within descriptor values in a
@font-palette-values rule. They are evaluated within the context of the root
element. Relative units are also evaluated within the context of the root element.
The @font-palette-values rule consists of the @font-palette-values at-keyword
followed by a block of descriptor declarations.
It has the following syntax:
The @font-palette-values rule accepts the descriptors defined in this specification.
If multiple @font-palette-values rules are defined with the same name,
the last one in document order wins, and all preceding ones are ignored.
Modify Bixa Color’s color palette.
--Cooler
Bixa
#7EB7E4
👁 overriding a color Bixa Color by Novo Typo. Above, using the colors in the font;
below, the orange changed to a blue.
Modify Handover Sans’s color palette
--Augusta
Handover Sans
--highlight
These descriptors apply solely within the context of the @font-palette-values rule in which they are defined,
and do not apply to document language elements.
When a given descriptor occurs multiple times in a given @font-palette-values rule,
only the last descriptor declaration is used
and all prior declarations for that descriptor are ignored.
Each of these palettes are applied
to each @font-face that share a family name.
I want to use two color fonts in a greenish palette.
The first font, Bixxxa, already has palette that I can use,
but for Bungeehee I need to overwrite one color
to get my green palette.
Note: In the case where a family name is shared
between multiple physical fonts,
(such as when a compound font is created, using unicode-range),
and if those fonts have different palettes,
the result of specifying a partial palette
using @font-palette-values
is unlikely to be what the author intended.
In this case,
it is better to supply a complete palette definition.
In the above example,
since two different font families have been set up
to have compatible named palettes,
one could go on to write
h3
Bixxxa Bungeehee
--ToxicGreen
Which would correctly apply the desired palette
in each font
even though they have different palette numbers.
A given set of @font-palette-values rules
define which author-defined palettes are available for use
within the documents that contain these rules.
An author-defined font color palette
must only be available to the documents that reference it.
Using an author-defined color palette outside of the documents that reference it
would constitute a security leak
since the contents of one page would be able to affect other pages,
something an attacker could use as an attack vector.
This at-rule follows the forward-compatible parsing rules of CSS.
Like properties in a declaration block,
declarations of any descriptors that are not supported by the user agent
must be ignored.
@font-palette-values rules require a font-family descriptor;
if it is missing,
the @font-palette-values rule is invalid and must be ignored entirely.
In cases where user agents have limited platform resources,
do not implement support for color fonts,
or implement the ability to disable color fonts,
@font-palette-values rules must simply be ignored;
the behavior of individual descriptors as defined in this specification should not be altered.
This descriptor defines the font families that this palette applies to,
using the same list of font families as § 5 Font Matching Algorithm.
This palette will only ever be applied to the fonts with these family names.
The value of this descriptor means that only named font families are allowed
and rules that include generic fonts in the list of font families
are syntax errors.
If syntax errors occur within the font family list,
the descriptor must be ignored
(will still be in the CSS OM, but will not match any font families).
9.2.2.
Specifying the base palette: the base-palette descriptor
Some color font formats include metadata
marking certain palettes as applicable on a light (close to white) background.
This keyword identifies the first available palette in the font file marked this way.
If the font file format does not account for this metadata,
or no palette in the font is marked this way,
this value behaves as 0.
dark
Some color font formats include metadata
marking certain palettes as applicable on a dark (close to black) background.
This keyword identifies the first available palette in the font file marked this way.
If the font file format does not account for this metadata,
or no palette in the font is marked this way,
this value behaves as 0.
Identifies a (zero-based) numerical palette index within the font.
Modify Banner Flag’s color palette
--Festival
Banner Flag
darkblue
--highlight
This descriptor specifies a palette in the font
which the containing @font-palette-values rule uses as an initial value.
If no override-colors key is present in the @font-palette-values rule,
then the @font-palette-values rule represents the palette in the font
with the same index as the value of this descriptor.
If a override-colors key is present in the @font-palette-values rule,
each item in the value of that descriptor overrides a single color
in the color palette represented by this @font-palette-values block.
Rename Handover Sans’s 3rd color palette
--Augusta
Handover Sans
If this descriptor is not present in the @font-palette-values,
or if the font does not contain a palette at the index of the value of base-palette,
it behaves as if 0 were specified.
If a font does not contain any color palettes,
no colors are included in the initial color palette
represented by this @font-palette-values rule.
Colors in the palette can be overridden
by using the override-colors descriptor in the @font-palette-values rule.
9.2.3.
Overriding a colors from a palette: The override-colors descriptor
This descriptor overrides colors
to the initial color palette represented by this @font-palette-values rule.
The override-colors descriptor takes
a comma-separated list of palette index entries and colors.
Each item in the comma-separated list represents a tuple of
an entry into the palette and a color to replace it with.
Each specified <color> must be an
absolute color,
otherwise the descriptor is invalid.
For each key/value pair in the value of this descriptor,
the color with that key in the initial palette (i.e. by using the base-palette descriptor)
is overwritten by the color specified in this descriptor’s value.
A key that is outside the range of indices of the initial palette is ignored,
but does not make the descriptor invalid.
Palette index entries
in the override-colors descriptor
are a (zero-based) palette index entry.
Integer values are zero-indexed.
If the keys of multiple distinct key/value pairs identify the same color index (either by name or by integer),
the last key is used for the purposes of rendering. However, for serialization purposes, both
key/value pairs are present.
Note: This means that using font-palette with the same value on two different elements
might result in different used palettes
because the value of variables inside the @font-palette-values rule
might apply differently in the context of those two elements.
Colors provided from CSS
(as overrides, or as new entries)
may use any supported colorspace.
Modify Blaka Ink’s color palette
--Festival
Blaka Ink
display-p3
prophoto-rgb
Note: The colors specified in versions 0 and 1 of the CPAL table are in sRGB.
9.3.
Selecting the text presentation style: The font-variant-emoji property
This property allows web authors to select
whether emoji presentation or text presentation is used
for certain emoji code points.
Traditionally, these presentation styles were selected
by appending Variation Selector 15 (U+FE0E) or Variation Selector 16 (U+FE0F)
to certain code points.
However, font-variant-emoji allows web authors to set a default presentation style
which can replace the variation selectors.
Only the code points listed by Unicode
as contributing to a Unicode emoji presentation sequence
are affected by this property.
Within this CSS specification,
these characters are referred to as Emoji Presentation Participating Code Points.
This property has no effect on any other characters.
This property is expected to affect font fallback;
however, the exact nature of the interaction of font fallback with font-variant-emoji
is explicitly unspecified.
However, a variation selector must be included in a previous cluster
as defined by the cluster matching section above.
A natural result of this behavior
is that a variation selector must not be rendered in a different font than the previous character.
It thus causes font fallback to behave
as if the respective variation selector was intended
(as in the values description);
then font fallback, including the cluster fallback rules, is performed,
which must take the presentation preference
of these "appended" variation selectors into account.
Even when font-variant-emoji is used,
the presence of Variation Selector 15 (U+FE0E) or Variation Selector 16 (U+FE0F)
in the contents of an element
override the rendering specified in font-variant-emoji.
Therefore, font-variant-emoji sets a default presentation
which the text being rendered can opt out of.
Note: Different platforms have different conventions about how to handle emoji presentation sequences.
A cross-platform UA might wish to follow the conventions of each individual platform,
or it might wish to use the same approach on all platforms.
When tasked with a request for an emoji style rendering,
a UA might wish to disregard fonts which do not include color tables.
A different UA might wish instead to use the same mechanical cluster fallback algorithm
that it would use for any arbitrary cluster.
Variation selectors other than FE0E VARIATION SELECTOR-15 and U+FE0F VARIATION SELECTOR-16
must not have any effect on font selection.
If one of these variation selectors is present,
but unsupported by the font previously selected,
the variation selector is ignored.
BCP47’s -u- extension to the language tag accepted by lang or lang should not be
considered when the user-agent decides whether to use emoji presentation or text presentation for a particular character.
normal
User agents can choose to draw a Emoji Presentation Participating Code Point
in either emoji style or text style.
User agents typically follow platform conventions when performing this decision.
Code points are rendered in accordance with [UTS51]
as either emoji-default, text-default, or text-only,
depending on the values of the Emoji and Emoji_Presentation
properties for each
Emoji Presentation Participating Code Point.
If present, FE0E VARIATION SELECTOR-15 and U+FE0F VARIATION SELECTOR-16
will override the default presentation of individual
Emoji Presentation Participating Code Points.
To show the emoji form of U+1F6CB COUCH AND LAMP,
let CustomEmoji.ttf obey the user agent’s notion
of supporting the emoji form of this character,
and use the following:
...
<div style=>🛋</div>
10.
Font Taxonomy
A given font may belong in one or more of the following categories:
10.1.
Installed Fonts
A font may be installed globally on a device. Such fonts are generally accessible in any application, even applications which have no concept of CSS. The file or files backing the font object exist on the user’s device, not as a remote resource.
Installed Fonts must not be Web Fonts, and Web Fonts must not be Installed Fonts.
10.2.
Web Fonts
A font may be backed by a remote resource, which must be requested using the user agent’s resource fetching infrastructure. Web Fonts are represented by one of two mechanisms:
A Web Font must not be accessible in any other Document from the one which either is associated with the @font-face rule or owns the FontFaceSet. Other applications on the device must not be able to access Web Fonts.
Installed Fonts must not be Web Fonts, and Web Fonts must not be Installed Fonts.
Web Fonts shadow Installed Fonts, so if an Installed Font has the same family name as a Web Font, the Installed Font is not accessible.
10.3.
Preinstalled Fonts and User-Installed Fonts
Users can choose to install fonts on their devices.
User-Installed Fonts are installed by an explicit action by the user,
such as clicking an "Install" button
or copying a file into a particular directory on their device.
Such fonts are User-Installed Fonts and also are Installed Fonts.
Web content authors should not count on the presence of user-installed fonts,
because there is no guarantee any user will have
performed the action to install a specific font.
Please see the Font Matching Algorithm
description of how user-installed fonts may interact with the font matching algorithm.
Any Installed Font which is not a User-Installed font is a Preinstalled Font. It is likely that all users of a particular version of a particular Operating System will have the same set of Preinstalled Fonts installed. As such, Web content authors targeting these Operating Systems may wish to use these fonts' family names inside font-family properties.
The features-opentype, features-aat and features-graphite techs
refer to support for font features,
commonly implemented in [OPENTYPE] with the
GSUB and the
GPOS tables,
as well as in [AAT-FEATURES] using the
morx and
kerx tables
and [GRAPHITE] with the
Silf,
Glat ,
Gloc ,
Feat and
Sill tables
as documented in the
Graphite Table Format.
The section on § 6 Font Feature Properties describes properties that interact with these facilities.
The color-colrv0, color-colrv1, color-svg, color-sbix and color-cbdt
technologies refers to various types of color font file technologies.
Each one represents a table
(COLR,
SVG,
sbix or
CBDT)
inside [OPENTYPE] or [AAT-FEATURES] fonts which must be supported
to satisfy this requirement.
The palettes tech refers to support for font palettes,
commonly implemented in the [OPENTYPE] and [AAT-FEATURES]
with the
CPAL table.
The section on § 9 Color Font Support describes properties that interact with these facilities.
The incremental tech
refers to client support for incremental font transfer [IFT].
Web authors can specify the tech() function
inside an @font-facesrc descriptor
to indicate that support is
required for correct rendering of a font.
This mechanism can be used for gracefully falling back
to an ancillary font
when requested support is not present.
This @font-face block shows how to use a color font if support is present on the user agent,
and falls back to a non-color-font if support is not present.
opentype color-COLRv1
opentype
11.2. Font formats
Format strings defined by this specification are as follows.
The <font-format> values are synonyms for the formats below.
Given the overlap in common usage between TrueType and OpenType,
the format hints "truetype" and "opentype"
must be considered as synonymous;
a format hint of "opentype" does not imply
that the font contains Postscript CFF style glyph data
or that it contains OpenType layout information
(see Appendix A for more background on this).
12.
Object Model
The contents of @font-face and @font-feature-values rules
can be accessed via the following extensions to the CSS Object Model.
12.1.
The CSSFontFaceRule interface
The CSSFontFaceRule interface represents a <@font-face> rule.
The list of one or more font families for which a given set of feature values is defined.
value maps of type CSSFontFeatureValuesMap, readonly
Maps of feature values associated with feature value names for a given font-variant-alternates value type
Each value map attribute of CSSFontFeatureValuesRule reflects the values
defined via a corresponding feature value block.
Thus, the annotation attribute
contains the values contained within a feature value block, the
ornaments attribute contains the
values contained with a feature value block and so forth.
The CSSFontFeatureValuesMap interface uses the
default map class methods
but the set method has different behavior. It takes a sequence of unsigned integers and
associates it with a given featureValueName. The method
behaves the same as the default map class method
except that a single unsigned long value is treated as a sequence of a
single value. The method throws an exception if an invalid number of
values is passed in. If the associated
feature value block
only allows a limited number of values, the set method
throws an InvalidAccessError exception when the input
sequence to set contains more than the limited number of
values. See the
description of multi-valued feature value definitions
for details on the maximum number of values allowed for a given type
of feature value block. The get
method always returns a sequence of values, even if the sequence only contains
a single value.
12.3.
The CSSFontPaletteValuesRule interface
[=]
: {
;
;
;
;
};
The fontFamily and basePalette interfaces are parsed according to the appropriate CSS property syntax.
13.
Serializing
13.1.
Serializing font-related properties
Unless specifically noted for individual properties,
the properties defined in this module
follow the principles of
CSSOM § 6.7.2 Serializing CSS Values.
13.2.
Serializing font-related at-rules
Unless specifically noted
for individual descriptors,
the descriptors defined for at-rules in this module
follow the principles of
CSSOM § 6.7.2 Serializing CSS Values.
In particular,
following the principle of shorter representation:
for descriptors which accept a range of values,
if the start and end values are the same
(the range is zero)
the descriptor is serialized as a single value, not a range.
For example, the rule
would serialize as
Also,
following the principle of retaining only the last-defined value,
as well as the principle of shortest representation,
multiply-specified tuples and multiple blocks
are serialized as a single block
containing only the last-defined value.
For example, the declaration:
foo
would be serialized as:
foo
Appendix A: Mapping platform font properties to CSS properties
This appendix is included as background for some of the problems and
situations that are described in other sections. It should be viewed as
informative only.
Font properties in CSS are designed to be
independent of the underlying font formats used;
they can be used to specify bitmap fonts,
Type1 fonts,
SVG fonts
in addition to the common TrueType and OpenType fonts.
But there are facets of the TrueType and OpenType formats
that often cause confusion for authors
and present challenges to implementers
on different platforms.
Originally developed at Apple,
TrueType [TRUETYPE] was designed as an outline font format
for both screen and print.
Microsoft joined Apple
in developing the TrueType format
and both platforms have supported
TrueType fonts since then.
Font data in the TrueType format consists of
a set of tables distinguished with common four-letter tag names,
each containing a specific type of data.
For example,
naming information,
including copyright and license information,
is stored in the name table.
The character map (cmap) table
contains a mapping
of character encodings to glyphs.
Apple later added additional tables
for supporting enhanced typographic functionality;
these are now called Apple Advanced Typography, or AAT, fonts.
Microsoft and Adobe developed
a separate set of tables for advanced typography
and called their format OpenType [OPENTYPE].
The OpenType specification is standardized at ISO as the
Open Font Format [OPEN-FONT-FORMAT].
In many cases the font data used under Microsoft Windows
or Linux
is slightly different from the data used under Apple’s Mac OS X
because the TrueType format allowed for
explicit variation across platforms.
This includes font metrics,
names and
character map data.
Specifically,
font family name data is handled differently
across platforms.
For TrueType and OpenType fonts
these names are contained in the name table,
in name records with name ID 1.
Multiple names can be stored for different locales,
but Microsoft recommends fonts
always include at least a US English version of the name.
On Windows,
Microsoft made the decision for backwards compatibility
to limit this family name to a maximum of four faces;
for larger groupings the
"preferred family" (name ID 16) or
"WWS family" (name ID 21)
can be used.
Other platforms such as OSX don’t have this limitation,
so the family name is used to define all possible groupings.
Other name table data provides names used
to uniquely identify a specific face within a family.
The full font name (name ID 4) and
the Postscript name (name ID 6)
describe a single face uniquely.
For example,
the bold face of the Gill Sans family
has a fullname of "Gill Sans Bold" and
a Postscript name of "GillSans-Bold".
There can be multiple localized versions of the fullname for a given face,
but the Postscript name is always a unique name
made from a limited set of ASCII characters.
On various platforms,
different names are used to search for a font.
For example,
with the Windows GDI CreateIndirectFont API,
either a family or fullname can be used to lookup a face,
while on Mac OS X the CTFontCreateWithName API call is used
to lookup a given face
using the fullname and Postscript name.
Under Linux,
the fontconfig API allows
fonts to be searched using any of these names.
In situations where platform API’s
automatically substitute other font choices,
it may be necessary to
verify a returned font matches a given name.
The weight of a given face can be determined
via the usWeightClass field of the OS/2 table
or inferred from the style name (name ID 2).
Likewise, the width can be determined
via the usWidthClass of the OS/2 table
or inferred from the style name.
For historical reasons
related to synthetic bolding at weights 200 or lower with the Windows GDI API,
font designers have sometimes skewed values in the OS/2 table
to avoid these weights.
Rendering complex scripts that use contextual shaping
such as Thai,
Arabic
and Devanagari
requires features present only in OpenType or AAT fonts.
Currently,
complex script rendering is supported
on Windows and Linux using OpenType font features
while both OpenType and AAT font features are used
under Mac OS X.
14.
Security Considerations
See items 9, 16 and 17 in the self-review questionnaire below.
15.1. What information might this feature expose to Web sites or other parties, and for what purposes is that exposure necessary?
This specification allows the use of Web Fonts,
which are requested on demand
but are not installed.
In the case where the document or stylesheet has a different origin to the Web Font,
this network request exposes information in the Referer header,
which may be harvested by font providers.
In addition to Web Fonts, this specification continues to allow
the use of Installed Fonts (both Preinstalled Fonts and User-Installed Fonts)
as introduced in CSS1.
While Web Fonts have the advantage of consistency across platforms,
Installed Fonts have the advantage of zero download time.
In some cases, and particularly for poorly-supported or minority languages,
Installed Fonts allow information to be displayed which could not otherwise be displayed
because there is no freely licensable Web Font which supports that language
or because the latency or download time would make it’s use infeasible,
particularly for languages with a large character repertoire,
or on slow or metered connections.
Note: The set of installed fonts
available in the Font Matching Algorithm
is explicitly undefined.
The available set of fonts is
used by trackers to fingerprint users and reduce their privacy.
However, some installed fonts,
even some user-installed fonts,
are required to make languages readable.
User agents may choose to make all installed fonts available
for language support and design integrity reasons,
or may choose to make some fonts unavailable for privacy reasons.
In addition, user agents may have additional facilities for fine-tuning this balance,
such as interfaces which prompt users to explicitly make
certain requested fonts available or unavailable
(perhaps on a per-site basis).
Different user agents, even running on the same Operating System,
are expected to strike different balances here.
In the case of user agents which perform rendering of local resources
(such as an HTML and CSS to PDF renderer,
or a Web-based wordprocessor)
access to all Installed Fonts
(both Preinstalled Fonts and User-Installed Fonts)
is necessary to provide the expected functionality.
An attacker may obtain fingerprinting information by querying the Installed Fonts.
In contrast to older technologies
(notably Adobe Flash, which provided a complete list of Installed Fonts
and sent this information in HTTP headers)
such probing must be done one font at a time,
providing the font family name
and then checking
(either via script,
or by using unicode-range to selectively download webfonts
depending on whether the user has a font by a certain name
that supports a certain character)
whether the font was loaded.
This takes time, and checking for more than a few hundred fonts
introduces a noticeable delay in page rendering.
For especially privacy-sensitive contexts,
options would include never downloading any webfonts
(at the risk that some characters may be rendered incorrectly, or not at all),
or always downloading all webfonts whether needed or not
(ignoring unicode-range,
and potentially downloading vast quantities of unused fonts
each time the page is viewed).
15.2. Is this specification exposing the minimum amount of information necessary to power the feature?
An emerging consensus is that a user agent must expose Preinstalled Fonts
for correct functioning,
but there is no consensus on exposing User-Installed Fonts.
This specification allows a user agent
to ignore User-Installed Fonts for the purpose of the Font Matching Algorithm.
Several existing user agents already do this.
A permissive amount of information exposes potentially more fingerprinting information;
a restrictive amount of information reduces fingerprinting but also reduces functionality
and in some cases, for minority languages, would break the Web completely for those users.
The possibility of a configurable, per-user opt-in to exposing some or all User-Installed Fonts,
or a per-origin opt-in, is being discussed.
The possibility of a privacy budget, which would penalize or disable
a malicious web page which tested a large number of fonts,
but allow a harmless page which tested a much smaller number,
has also been discussed.
Some user agents expose a more restricted set of Preinstalled Fonts
in their Private Browsing, Incognito, or Resist Fingerprinting modes,
compared to their normal mode.
15.3. How does this specification deal with personal information or personally-identifiable information or information derived thereof?
Personal information is not exposed by this specification.
Personally identifiable information may be exposed in some cases.
For example, for someone in Japan, having conditionally-enabled Japanese fonts
enumerable probably isn’t a substantial fingerprinting vector.
For someone in Europe who has a Japanese IME in the text input menu, they are.
15.4. How does this specification deal with sensitive information?
Fingerprinting on Installed Fonts may expose sensitive information in some cases.
For example, persecuted minorities risk leaking sensitive information
by exposing that they use fonts required for a persecuted minority language;
either by requesting a Web Font from a third party service,
or exposing Preinstalled Fonts or User-Installed Fonts associated with that language.
15.5. Does this specification introduce new state for an origin that persists across browsing sessions?
Similarly, font palette values
must only be available to the documents that reference it.
Using an author-defined color palette outside of the documents that reference it
would constitute a security leak since the contents of one page
would be able to affect other pages,
something an attacker could use as an attack vector.
15.6. What information from the underlying platform, e.g. configuration data, is exposed by this specification to an origin?
The system-ui keyword exposes the operating system’s default system UI font to fingerprinting mechanisms.
15.7. Does this specification allow an origin access to sensors on a user’s device
No.
15.8. What data does this specification expose to an origin? Please also document what data is identical to data exposed by other features, in the same or different contexts.
For third-party Web Fonts loaded via a stylesheet,
the stylesheet origin may be exposed to the third party in the Referer header.
In addition, careful pairing of unicode-range and distinct src urls
allows the third-party to see which characters are used on a page,
which is a privacy risk for large character repertoire scripts
such as CJK.
For third-party Web Fonts preloaded in the HTML,
the document origin may be similarly exposed.
15.9. Does this specification enable new script execution/loading mechanisms?
No.
Specifically, for SVG-in-OpenType color fonts,
the SVG used for glyph definitions should not contain script elements,
and any script elements that do occur will not be executed.
15.10. Does this specification allow an origin to access other devices?
No.
15.11. Does this specification allow an origin some measure of control over a user agent’s native UI?
There is some risk that an attacker can spoof a native UI feature
by determining the Operating System
and using native-looking fonts appropriate to that Operating System.
15.12. What temporary identifiers might this specification create or expose to the web?
None.
15.13. How does this specification distinguish between behavior in first-party and third-party contexts?
For font loads, user agents must use the
potentially CORS-enabled fetch method defined by the [HTML] specification
for URLs defined within @font-face rules.
When fetching, user agents must use "Anonymous" mode,
set the referrer source to the stylesheet’s URL
and set the origin to the URL of the containing document.
Thus, fonts will typically not be loaded cross-origin
unless authors specifically take steps to permit cross-origin loads.
15.14. How does this specification work in the context of a user agent’s Private Browsing or "incognito" mode?
The specification makes no distinction.
Some user agents may expose a more restricted set of Installed Fonts in these modes.
15.15. Does this specification have a "Security Considerations" and "Privacy Considerations" section?
Yes.
15.16. Does this specification allow downgrading default security characteristics?
No.
15.17. What should this questionnaire have asked?
It should have asked whether a malicious payload could crash the application,
or indeed the entire Operating System,
or even cause remote code execution.
This possibility does exist for suitably crafted fonts
on some platforms
when fonts are installed and rendered,
and this has been exploited in the wild.
In practice, user agents running on Operating Systems with this vulnerability
use a font sanitizer to detect such malformed or malicious fonts
and prevent their being used.
16.
Accessibility Considerations
The use of fonts to provide a visual rendering of text should not, in general, impact accessibility.
For example, people using a screen reader to render text to speech will not download fonts,
and are unaffected by what those fonts would have contained.
However, this assumes that the semantics conveyed by the font glyphs
and the semantics conveyed by the characters
are the same.
Historically, this has not always been the case.
For example, in the early days of the Web it was common to use fonts
(such as "Symbol", though others were used)
to make Latin letters have Greek glyphs;
while this worked visually,
it would not work with a screen reader
and text was also hard to search or index
because the mapping was font-specific.
With the rise of Unicode, it is now standard practice to use Greek characters for Greek text,
and for Greek glyphs in fonts to map to Greek characters.
Sadly, but avoidably, this practice persists
with badly designed icon fonts.
For example, such a font might put a "printer" icon on the Latin letter "P".
This practice scatters meaningless letters through the text,
which negatively affects text searching and indexing,
gives hard to understand rendering if the icon font does not load,
and impedes screen readers.
A well designed font must assign such icons to semantically meaningful characters.
For example, the printer icon might be assigned to the string "printer"
or to the Unicode character 🖨 U+1F5A8 (PRINTER).
Munira Tursunova and Dominik Röttsches developed the features for animating font-palette.
John Hudson was kind enough to take the time to explain the subtleties of OpenType language tags
and provided the example of character variant usage for displaying text on Byzantine seals.
Elika Etemad supplied some of the initial design ideas for the '@font-feature-values' rule.
Special thanks to Tab Atkins Jr.
for providing the text for the section on Font Rendering Controls
as well as the section on the font-display descriptor.
Special thanks to Ilya Grigorik and David Kuettel for their help in developing these sections.
In font matching algorithm, distinguished use of varfont slnt axis from synthetic obliquing
Clarified that font variations do not count as font synthesis
Clarified the effect of auto values on @font-face is to select the appropriate "normal" value
Improved font fallback on font-variant-emoji with variation selectors
Clarified that the oblique angle must be the same sign as requested
Defined synthetic oblique in vertical text
Used CSS Color 5 absolute color definition for override-colors
Made font-stretch a legacy name alias for font-width
The palette-mix() grammar requires color-interpolation-method, so removed leftover prose defining what to do if it is omitted
Add more generics samples, for fang song and kai
Listed fonts used for Urdu samples
Users must be able to modify the set of installed fonts available for rendering web pages
Added examples of Urdu text in nastaliq and in fallback naskh fonts
Clarified that keywords inside local() are an error
Clarified that the system-ui generic font family may be composite, and is affected by unicode coverage and content language
Removed culturally specific claims that particular typographic styles universally convey concepts such as formality or playfulness
Added a new generic(nastaliq) family, for Urdu, Persian, and other languages
Added a new generic(kai) family, for Simplified and Traditional Chinese
No longer claim that cursive and kaiti have identical meanings
Added a note on the historical reasons for the serif and sans-serif generic font families
Dropped the emoji generic font-family, because the font-variant-emoji property covers the desired functionality better
Added Internet Media Types to the table of font formats
Moved the consequences of installed fonts out of Taxonomy and into Font Matching Algorithm
Clarified that the font-family and src descriptors are no longer required for validity, as they can be added later by script.
Consistently use generic(fangsong) instead of fangsong, as the generic font family name. Note that it is script-specific
Defined a third type of font family name, <system-family-name> which has some constraints on how it can be specified
Added a missing at-keword in the production of @font-feature-values
Refactored the @font-feature-values grammar
Restored some acknowlegements which had been dropped when porting CSS Fonts 3 into this specification
Redefined the grammar of the src attribute to use a <font-src-list> production, rather than "see prose"
Deleted broken link to an informative reference which no longer exists
Described three classes of generic font families, including whether they are writing system (script) specific and whether they are always required to match to a locally installed font
Added a new functional notation, generic(), for new generic font families
Added a clarifying note about platform API limitations which might restrict color font rendering to sRGB
Removed leftover, no-longer applicable mention of "forcing colored glyphs not to be used"
Refined the font-palette computed value
Added an example of Nabla font with an animated palette
Described modified font-palette syntax and rules for interpolation
and percentage normalization, referencing the CSS Color specification
Clarified that the used color-scheme affects font-palette: normal
Added missing numeric range annotations for slope angle and font-feature-settings integer
Defined a production for <family-name>
For locally installed fonts, used local(<family-name>)
Fixed ambiguous descriptor auto-links
Fixed markup of the <family-name> production
Defined feature and variation names with <opentype-tag> rather than bare strings
Clarified that general math functions (not just calc) are allowed in @font-palette-values descriptors
Added the font-synthesis-position property
Added an example of a non-sRGB color in override-colors
Use rule-list rather than stylesheet in productions
Added URL request modifiers for crossorigin, referrerpolicy & integrity
Aligned the incremental tech keywords to match the latest Incremental Font Transfer specification
Use last-defined value for font-feature-values
font-feature-values and font-feature-settings are now de-duplicated, and sorted
Suggested user agents prompt users to make certain user-installed fonts available, or unavailable
Improved the Privacy Considerations, balancing privacy and legibility trade-offs for user-installed fonts
Clarified the font shorthand, using the terms set explicitly, reset implicitly and cascade implicitly
Clarified shorter representation for zero ranged descriptors
Tightened definition of first available font
Added emoji as a keyword to unicode-range
Allowed implementations to ignore requests to turn off font features which the OpenType specification says are always required
Gave clearer advice to authors about font-feature-settings and font-variation-settings
Readability improvements to the grammar for font-variant sub-properties
Corrections to some examples so they again match the specification
Clarified what it means to ignore an invalid font-family descriptor
Allowed the font-family descriptor of @font-palette-values to take a list
Added Web Platform Tests coverage
font-palette and '@font-palette-values' no longer at risk
Allowed limited set of string-form format specifies, for Web compatibility
Make feature-* features-* for consistency
Changed initial value of font-variant-emoji from auto to normal, for consistency with other font-variant-* properties
Added values from font-variant-emoji to font-variant shorthand
Added canonical list of font-variant-* properties affected by font-variant shorthand
Renamed "system font fallback" to "installed font fallback to avoid confusion with system font
Updated property and descriptor values to use the range notation
Fixed some examples which did not match the specification
Clarified that color fonts are not enabled by default
Added some more explanation to font-palette:light|dark
Use last declaration of key/palette index in override-colors
Removed currentColor from override-colors example
Clarified that out of range palette indexes are ignored
This summarizes the changes in CSS Fonts 4, compared to CSS Fonts 3.
Added font-display descriptor for @font-face
Added @font-feature-values rule
Added font variation properties, including font-optical-sizing and font-variation-settings
Added color font support
Defined feature-value-name for font-variant-alternates
Added new auto value for font-weight, font-style and font-stretch descriptors
Added ui-serif, ui-sans-serif, ui-monospace, and ui-rounded generic font families
Added font-size: xxx-large
Added font metric override descriptors
Added some motivation for generic font families
Added font feature technology keywords opentype, aat, and graphite
Added math value to font-size
Conformance
Document conventions
Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words “MUST”,
“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.
All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with class=,
like this:
This is an example of an informative example.
Informative notes begin with the word “Note” and are set apart from the
normative text with class=, like this:
Note, this is an informative note.
Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <strong class=>, like
this:
UAs MUST provide an accessible alternative.
Conformance classes
Conformance to this specification
is defined for three conformance classes:
A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.
A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.
Partial implementations
So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers must
treat as invalid (and ignore
as appropriate) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents must not selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
be ignored.
Implementations of Unstable and Proprietary Features
Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.
normal | light | dark | <palette-identifier> | <palette-mix()>
normal
all elements and text
yes
N/a
by computed value
per grammar
specified keyword, identifier or <palette-mix()> function.
<palette-mix()> must be simplified to a single keyword or identifier if resulting palette is equivalent.
What direction should positive and negative obliques skew in vertical writing mode?
How do we achieve skews in the opposite dimension
(needed for vertical writing)? ↵
fallback and optional can result in some faces in a family being used
while others are required to fallback,
giving a "ransom note" look.
Perhaps require that all fonts in a family have the same behavior (all swapped in, or all fallback)?
See also the @font-feature-values for controlling the behavior on a font family basis. ↵
