The Parts of a CSS Rule All rules, regardless of their locations or types, have the following structural elements: n Selectors are the alphanumeric characters that identify a rule. A selector can be an HTML tag selector, a class selector, an ID selector, a universal selector or a combination of those basic selectors to create context based styles. Properties identify what is being defined. Several dozen properties are available; each is responsible for an aspect of the page content’s behavior and appearance. Values are assigned to a property to define its nature.