W3.org:

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

CSS1 Specification
World Wide Web Consortium's CSS Level 1 recommendation.
http://www.w3.org/TR/REC-CSS1/
Top/Computers/Data_Formats/Style_Sheets/CSS/Reference/Specifications

Multi-column layout in CSS
CSS3 working draft for multi-column layout. [Not final specification]
http://www.w3.org/TR/css3-multicol/
Top/Computers/Data_Formats/Style_Sheets/CSS/Reference/Specifications

CSS Namespace Enhancements (Proposal)
CSS3 working draft for compatibility with XML namespaces. [Not final specification]
http://www.w3.org/TR/css3-namespace/
Top/Computers/Data_Formats/Style_Sheets/CSS/Reference/Specifications

Behavioral Extensions to CSS
CSS3 working draft to allow event handlers to be given in CSS and include Microsoft's HTML components. [Not final specification]
http://www.w3.org/TR/becss/
Top/Computers/Data_Formats/Style_Sheets/CSS/Reference/Specifications

Paged Media Properties for CSS3
Draft for CSS printing features. [Not final specification]
http://www.w3.org/TR/css3-page/
Top/Computers/Data_Formats/Style_Sheets/CSS/Reference/Specifications

User Interface for CSS3
Draft for new CSS properties mostly related to styling forms and interactive user interfaces. [Not final specification]
http://www.w3.org/TR/css3-userint
Top/Computers/Data_Formats/Style_Sheets/CSS/Reference/Specifications

CSS3 module: W3C Selectors
Draft for selectors in CSS3. [Not final specification]
http://www.w3.org/TR/css3-selectors/
Top/Computers/Data_Formats/Style_Sheets/CSS/Reference/Specifications

Introduction to CSS3
Draft describing the modularization of CSS3. [Not final specification.]
http://www.w3.org/TR/css3-roadmap/
Top/Computers/Data_Formats/Style_Sheets/CSS/Reference/Specifications

CSS2 Specification
The official Cascading Style Sheets 2 specification, presented by the World Wide Web Consortium. A comprehensive standards document outlining what CSS2 is with definitions and explanations of all its properties and terminology.
http://www.w3.org/TR/CSS2/
Top/Computers/Data_Formats/Style_Sheets/CSS/Reference/Specifications

W3C CSS Validator
Finds errors and potential problems in Cascading Style Sheets.
http://jigsaw.w3.org/css-validator/
Top/Computers/Data_Formats/Style_Sheets/CSS/Tools