CSS, Cascading Style Sheets, was first proposed in 1994. The first CSS Standard, produced by the World Wide Web Consortium (W3C) in 1996. There have been three major versions of CSS; CSS Level 1, CSS Level 2, and CSS Level 3. Each version builds on the works of the previous levels. HTML defines the content and its structure. CSS defines how that structure is to be presented.
CSS Property Reference
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HTML Help by the Web Design Group
Chock full of references, tools and help.
The WebKit Open Source Project
W3C Cascadeing Style Sheets, level 1
Cascading Style Sheets, level 1. Recommended 17-December-1966, Revised 11-April-2008
W3C Cascading Style Sheets
W3C Cascading Style Sheets Home Page. Links from this page to CSS Level 3 stable draft documents.
W3C Cascading Style Sheets, Level 2
Cascading Style Sheets, level 2. CSS2 Specification. Recommendation 12-May-1998
W3C Style Sheets
Web Style Sheets Home Page.
Web Browser Standards Support
This document will summarize the level of support for web standards and maturing technologies in popular web browsers. It covers the Internet Explorer, Firefox, and Opera web browsers, with focus on the HTML, CSS, DOM, and ECMAScript technologies.
Web Site Work Station
Turorials for HTML, CSS and Website Design.