CSSTidy

CSSTidy is an opensource CSS parser and optimiser.

W3Schools - CSS Tutorial

In our CSS tutorial you will learn how to use CSS to control the style and layout of multiple Web pages all at once.

W3C - Web Style Sheets CSS tips & tricks

A random collection of CSS examples and some help in using them.

www.ericmeyeroncss.com

One of many GURUs on CSS.

A List Apart

A List Apart Magazine explores the design, development, and meaning of web content, with a special focus on techniques and benefits of designing with web standards.

webstandards.org

The Web Standards Project is a grassroots coalition fighting for standards that ensure simple, affordable access to web technologies for all.

www.zeldman.com

Author of "Designing with Web Standards - The definitive guide to understanding and implementing web standards."

WC3 - XHTML

XHTML^™ 1.0 The Extensible HyperText Markup Language (Second Edition)

W3 Schools - XHTML

In this tutorial you will learn the difference between HTML and XHTML. You will also learn how this Web site was converted to XHTML.

mootools.net

MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer. It allows you to write powerful, flexible, and cross-browser code with its elegant, well documented, and coherent API.