In an HTML document, CSS Class selectors match an element based on the contents of the element's class attribute. The class attribute is defined as a space-separated list of items, and one of those items must match exactly the class name given in the selector. The selector is written by first using the node name of the element, followed by a period, then the name of the class. Example: span.class1. It can also be written simply by stating the class name, without having to have it match an exact element. Example: .class1. Example: