This project is currently unmaintained, and the CSS Syntax spec has dramatically shifted since I last updated. If you would like to use the current parser, find the April 1 2013 commit, as it's the last good commit. The parser is still mostly correct at that point; in general, it's far more correct than virtually any other JS parser in use.
I plan to update this to the current CSS Syntax spec soonish, but don't know exactly when that will be.
This project implements a standards-based CSS Parser. I'm writing the CSS Syntax spec http://dev.w3.org/csswg/css3-syntax, and need an implementation of it for testing purposes.
This parser is not designed to be fast. Its structure and coding style are instead meant to be very close to the spec, so that it's easy to verify that the code matches the spec (and vice versa) and to make it easy, when the spec changes, to make the same change in the parser.
However, it may still be useful to someone in production, as it is intended to fully and completely match browser behavior (at least, as much as the final spec does).
The functionality is spread across two files for organizational purposes. The tokenizer.js file comes first - call the tokenize() function with a string containing CSS, and it will return an array of CSS tokens. parser.js comes next - call parse() with an array of CSS tokens, and it will return a stylesheet object implementing a very simple tree of rules.
Simon Sapin added some CommonJS export statements to the bottom of tokenizer.js and parser.js exporting the tokenize and parse functions, so as far as I know it will Just Work(tm). I have no idea how to fix it if anything goes wrong, though.