The vast majority of CSS is concerned with styling documents, but it offers a passel of useful interface-styling tools for more than just documents. For example, Mozilla's developers created its browser's interface (and that of many Mozilla clones) using a language called XUL. XUL employs CSS and CSS-like declarations to present the navigation buttons, sidebar tabs, dialog boxes, status boxes, and other pieces of the chrome itself.
Similarly, you can reuse aspects of the user's default environment to style a document's fonts and colors; it's even possible to exert influence over focus highlighting and the mouse cursor. CSS2's interface capabilities can make the user's experience more enjoyableor more confusing, if you aren't careful.