CSS References

Cascading Style Sheets, Level 1: The W3C's Recommendation gives the complete specification for CSS1.
Cascading Style Sheets, Level 2: CSS2 builds on CSS1 and most valid CSS1 style sheets are valid CSS2 style sheets.
Cascading Style Sheets, Level 2 Revision 1 (CSS 2.1): CSS 2.1 corrects a few errors in CSS2, while removing some CSS2 features (most with poor browser support) and adding other features (most with good browser support and strong demand). CSS 2.1 is intended to replace CSS2, and represents a "snapshot" of CSS features that were implemented interoperably when it was published.
Cascading Style Sheets: The W3C's CSS home with links to CSS news, tutorials, browsers, and authoring tools.
CSS Frequently Asked Questions: A list of frequently asked questions concerning CSS.
The House of Style: Includes a CSS handbook and tutorial, and a detailed look at browser support.
Hide CSS from Browsers: Tricks to hide CSS rules from buggy browsers.