W3.org:

W3.org has 287 URLs listed in 163 Dmoz categories.

JSSS: JavaScript-Based Style Sheets
Overview of Netscape's proposed JavaScript based style system. Historical.
http://www.w3.org/Submission/1996/1/WD-jsss-960822
Top/Computers/Data_Formats/Style_Sheets

W3C - Cascading Style Sheets
Authoritative information from the W3C. Provides complete specifications, lists of CSS browsers and authoring tools, and information about learning CSS.
http://www.w3.org/Style/CSS/
Top/Computers/Data_Formats/Style_Sheets/CSS

Cascading Style Sheets: Celebrating 10 Years with Style
W3C mini-site to mark the 10-year anniversary of the first published CSS recommendation. Includes time-line, hall of fame, essays, and a gallery.
http://www.w3.org/Style/CSS10/
Top/Computers/Data_Formats/Style_Sheets/CSS

W3C CSS Working Group Blog
News, announcements, and opportunities for discussion from the group working on the next CSS specification.
http://www.w3.org/blog/CSS?tempskin=_atom
Top/Computers/Data_Formats/Style_Sheets/CSS

Learning CSS: Books
List of books on CSS from the W3C's CSS page.
http://www.w3.org/Style/CSS/learning#books
Top/Computers/Data_Formats/Style_Sheets/CSS/Books

W3C Core Styles
Ongoing project exploring shared style sheets over the Web. Includes several layout designs and details on implementation.
http://www.w3.org/StyleSheets/Core/
Top/Computers/Data_Formats/Style_Sheets/CSS/Examples

Adding a Touch of Style
Guidance on the basics of adding simple CSS to a site. Also includes a HEX/RBG color converter and color chart.
http://www.w3.org/MarkUp/Guide/Style
Top/Computers/Data_Formats/Style_Sheets/CSS/FAQs,_Help,_and_Tutorials/Tutorials

CSS Home Page
World Wide Web Consortium's CSS Page that includes links to standards and other reference pages. Also includes news on CSS happenings.
http://www.w3.org/Style/CSS/
Top/Computers/Data_Formats/Style_Sheets/CSS/Reference

CSS ICC Profile Support Proposal
Working draft to give CSS finer control of reproducibility and accuracy of colors. [Not final specification]
http://www.w3.org/TR/css3-iccprof
Top/Computers/Data_Formats/Style_Sheets/CSS/Reference/Specifications