Cascading style sheets (CSS) are a powerful mechanism to customize HTML rendering inside a Web browser. The CSS2 specification comes from the W3C, and has now reached the status of a W3C recommendation.

The CSS syntax is a great improvement over the .Xdefault resource mechanism of the X Window System. The basic idea remains the same: matching a pattern and setting resource values. CSS is devoted to HTML rendering, matching HTML tags and setting style values. XML is another CSS target, especially as used within the SVG (Scalable Vector Graphics) recommendation from the W3C.