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.