Validate HTML, XHTML, XML, etc.

W3C Markup Validation Service

Markup Validation Service User Guide

Web Design Group HTML Validator

W3C CSS Validation Service

W3C Link Checker

HTML 4.01 Specification (W3C)

XHTML 1.0 Specification (2nd edition)

Cascading Style Sheets, Level 1

Cascading Style Sheets, Level 2

Web Accessibility Initiative (WAI)