Building web pages with XHTML 1.1

Sometimes you want to change the appearance of certain elements on your page using a CSS stylesheet (see Section 5, “Separating content and presentation with CSS”). To do this, invent a name for those elements; this name must conform to the rules for names discussed in Section 6.2, “The ID datatype”. Then, add a class attribute to those elements.

For example, suppose several paragraphs on your pages are important warnings, and you want to print them in larger type with a red border. To do this, add an attribute “class='warning'” to each paragraph. Then, your CSS stylesheet would use the selector p.warning with a rule that specifies larger type and a red border.

Any number of elements can have the same value of their class attributes, unlike id values which must be unique within a document (see Section 15.4, “The id attribute: Assigning a unique identifier to an element”).

An element can actually belong to several classes. In that case, use a class attribute whose value is a list of its classes separated by spaces. For example, the start-tag <ul class="info Kyzyl Feynman" would mean that the ul element is a member of three different classes.