YUI Compressor is JavaScript and CSS minificator.

YUI Compressor is written in Java and relies on Rhino to tokenize the
source JavaScript file.  It starts by analyzing the source JavaScript
file to understand how it is structured.  It then prints out the token
stream, omitting as many white space characters as possible, and
replacing all local symbols by a 1 (or 2, or 3) letter symbol wherever
such a substitution is appropriate (in the face of evil features such as
eval or with, the YUI Compressor takes a defensive approach by not
obfuscating any of the scopes containing the evil statement).

The CSS compression algorithm uses a set of finely tuned regular
expressions to compress the source CSS file.  The YUI Compressor is
open-source, so don't hesitate to look at the code to understand exactly
how it works.