Validating and testing a website is critical for code debugging, checking for compliance with web standards and web accessibility, usability, readability, speed, performance, compatibility, and user experience.

This is not a complete list and I’m adding to it all the time.

Web Standards Site Validation

• The W3C Markup Validation Service
• Unicorn
• Dr. Watson’s site validation check
• Site Check: Website Test Tests Multiple Validators – UITest.com
• The W3C CSS Validation Service
• Juicy Studio: CSS Analyser
• Web QA Test Tool Links
• HTML Validator
• WDG HTML Validator
• The Ultimate Testing Checklist – SitePoint

Cross Browser and Mobile Testing

• Check Browser Compatibility, Cross Platform Browser Test – Browsershots
• IE NetRenderer – Browser Compatibility Check
• mobiReady – dotMobi compliance & mobileOK checker
• W3C mobileOK Checker
• ViewLikeUs – Check Sites in Various Resolutions!
• My DebugBar – IETester / Browser Compatibility Check for Internet Explorer Versions from 5.5 to 10

Speed and Performance Testing

• Pingdom tools
• Website Speed Test – iWEBTOOL.com
• The Webmaster’s Turbo Kit: 50-Plus Tips and Resources to Improve Your Site’s Speed and Performance – Inside CRM
• Web Page Analyzer – Web Site Optimization
• Load Impact – Online Load Testing and Performance Testing Service
• Yahoo! YSlow for Firefox Performance Tester
• Robots checker

Link Testing

• W3C Link Checker
• URL checker

Usability

• Web Design Theory: Legibility and Readability – KLIF Media
• Usabilla – web design usability feedback
• Now You See Me – A List Apart
• The Essential Website Usability Checklist – Daily Bits
• The Blog Usability Checklist – Skelliewag.org
• Usability Toolkit
• A UK Usability Professionals Association guide – Key questions to ask your usability testing supplier
• 25-point Website Usability Checklist
• 247 web usability guidelines
• Checklist for Usability Forms – Website Usability
• Everyday Usability – 14-Point Checklist for Success – Website Magazine
• 10 Free Web-Based Web Site Accessibility Evaluation Tools – Usability Geek

XML, RSS, and Feed Validation and Testing

• W3C Feed Validation Service
• XML Well Checker and Validator