👁 Third International World-Wide Web Conference
SIMPLE STYLE SHEETS
FOR SGML & HTML
ON THE WEB
World-Wide Web conference spring 1995
Developer's day --- 14 April 1995
Darmstadt
Bert Bos
Dept. of Humanities Computing
University of Groningen
👁 Image
Context: Argo browser
Based on W3A ()
- HTML viewer applet
- inline images handled by different applets
- upgrading to new formats is cheap
The HTML applet
- real-time ``live'' formatting
Seems to be the trend: Netscape, HotJava, Arena
- implementation:
FormattedText widget --
SimpleSGML widget --
HTML2 widget
(widgets will become public in a few days)
Widget hierarchy (1)
FormattedText
- list of ``chunks'' (atomic units): words, hor. spaces (fixed or
strechable), vert. spaces, parshapes, inline widgets
- 3 ``tracks'': main, left floats, right floats
- NB. nothing specific to SGML/HTML
SimpleSGML
- parser for simple SGML (``canonical form'', see Sperberg-McQueen
& Goldstein)
- parser for stylesheets (actually Xlib/Xrm does most of the work)
- uses stack of styles, produces sequence of ``chunks''
Widget hierarchy (2)
HTML2
- overrides some methods:
- heuristic rules for omitted tags
- more entities (WWW icon entities) [nyi]
- forms (as inline objects) [nyi]
- currently accepts a mixture of HTML2 and HTML3
- NB. does no parsing
Assumptions (1)
- Addressable units
- whole elements only
- by context or by ID
- style sheet is static (declarative)
- style sheet consists of rules; rules
consist of: selector + property + value;
value may be an expression
- attributes not in selector, but in expression
Assumptions (2)
- text is rendered roughly in the order it is received
(exception: tables, math)
- only appearrance specified, not interaction
- SGML, not just HTML
Stream-based SS vs DSSSL
- Complementary!
- DSSSL needed for more complex layouts (re-arranging of text,
columns)
- 2 (or more) links in a document?
User preferences
Browser's default style | document's style | user's style
Select 1, 2 or all 3
Combined just like X resources (override, replace, merge)
- pro:
- flexible
- already widely used
- con:
- not always intuitive
- maybe too flexible?
Style properties (1)
Some selected properties
empty ()
- needed by parser (omitted end tag)
emphasis ()
- level of emphasis (at least 0, 1 2)
- allows browser to choose best method
- is this needed?
Style properties (2)
background ()
- behind text only
- looks funny for longer, unjustified text
- maybe need separate background for paragraphs?
prebreak, postbreak ()
- break line, then insert vert. whitespace
- measured in ``standard line heights''
- maybe add units (mm, ex, pixels)?
Style properties (3)
leftindent, rightindent ()
- added to inherited margin
- measured in ``standard em'' (rel. to default font)
- measured from edge, discarding floats
track ()
- 3 tracks: main, left/right float areas
- do we need more? (footnote, running head)
- NB ``title'' is not a track
Style properties (4)
minimized ()
- contents are ``folded'' or put into pop-up
- different from hide
keepwidth (
- tries to determine width of float from previous line
- not completely satisfactory: caption cannot precede figure
Style properties (5)
id ()
- assigns ID to an element
- usually taken from attribute
- allows assigning styles to individual element instances
insertbefore, insertafter ()
- insert arbitrary text
- can be used to define new tags
math
- is currently unspecified :-(
Expressions
Currently not very well developed
- IDs
-
an EM within an elt. with ID = ``par12''
- attributes
-
value taken from ALIGN attribute
- conditional
-
2nd arg. is reg. exp.
More general expressions needed (in particular concatenation)
External influences
Cf. Cascading stylesheets and DSSSL (``length spec'')
- width of window
- age of document
- number of available colours
- user's history
- ...
Currently none specified
Odds & ends
&sbsp; (soft break space)
- breakable, but invisible (cf. ­)
- in Unicode, but not in ISO :-(
- replaces Netscape's
External vs in-line style sheets
- vs
- priority?
- in-line handy for one-time styles
Other output formats
- Postscript, TeX, plain text,...
