Technical Tips: IDs, Classes

• 1 ID per page, many classes per page. “Check your IDs: Only one element in a document can have a certain value for the id attribute, while any number of elements can share the same class name. [..] Class and id names can only consist of the characters [A-Za-z0-9] and hyphen (-), and they cannot start with a hyphen or a digit (see CSS2 syntax and basic data types).” [Roger Johansson]
• Element names in selectors are case sensitive. “Remember case sensitivity. When CSS is used with XHTML, element names in selectors are case sensitive. To avoid getting caught by this I recommend always using lowercase for element names in CSS selectors. Values of the class and id attributes are case sensitive in both HTML and XHTML, so avoid mixed case for class and id names.” [Roger Johansson]
• CSS classes and IDs must be valid. “I.e. beginning with a letter, not a number or an underscore. IDs must be unique. Their names should be generic, describe functionality rather than appearance.” [CSS Best Practices]
• You can assign multiple class names to a given element. “You can assign multiple class names to an element. This allows you to write several rules that define different properties, and only apply them as needed.” [Roger Johansson]

Kaynak: Smashingmagazine