Which characters are valid in CSS class names/selectors?

It is very easy to choose a valid class name or selectors in CSS just follow the rule. 

• A valid name should start with an underscore (_), a hyphen (-) or a letter (a-z)/(A-Z) which is followed by any numbers, hyphens, underscores, letters.
• It cannot start with a digit, starting with the digit is acceptable by HTML5 but not acceptable by CSS.
• Two hyphens followed by a number is valid.

Example 1: This example describes the list of valid id selectors using CSS. 

Output:

Example 2: This example describes the list of valid class selectors using CSS. 

Output: