Types of CSS Rules The best thing about cascading style sheets is that they are amazingly simple to set up. They don’t require plug-ins or fancy software—just text files with rules in them. A CSS rule defines what the HTML should look like and how it should behave in the browser window. CSS rules come in three types, each with specific uses: -HTML selector. The text portion of an HTML tag is called the selector. For example, h1 is the selector for the tag. The HTML selector is used in a CSS rule to redefine how the tag displays. -Class. A class is a “free agent” rule that can be applied to any HTML tag at your discretion. You can name the class almost anything you want. Because a class can be applied to multiple HTML tags, it is the most versatile type of selector. -ID. Much like class selectors, ID rules can be applied to any HTML tag. ID selectors, however, should be applied only once to a particular HTML tag on a given page to create an object for use with a JavaScript function.