Cascading Style Sheets Level 1: World Wide Web Consortium's CSS Level 1 recommendation

Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) 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

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

CSSCheck: A cascading style sheet lint to check the style sheet's syntax, style, and accessibility

W3C CSS Validator: Finds errors and potential problems in Cascading Style Sheets

WestCiv: The Complete CSS Guide: Five-part tutorial for understanding and using Cascading Style Sheets. Covers introductory material, the features and properties of CSS, and real-world issues, such as building for compatibility and accessibility