Site Admin Stuff

Stuff I depend on as a webmaster and web developer

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Validation, Cross-browser checking

Make sure it validates and works in most browsers. See also the Mobile accessibility section for phone/pda-specific validators.
HTML 2 Txt See what your website looks like to a text-only browser such as lynx and some search engine spiders
W3C HTML validator W3C HTML validator
Jigsaw Jigsaw W3C CSS Validator
Backwards Compatibility Checker Backwards Compatibility Checker Delorie's tool to see what your page looks like with various browser features disabled (Frames, JS, CSS etc)
Header Checker See what HTTP headers your site sends out, check for correct mime types, 301 redirection etc
Walidator Validate CSS, HTML, RSS, RDF, XHTML, etc
WebXCACT WebXCACT Test your site for quality, accessibility, and privacy issues
Favicon validator Favicon validator Make sure your favicon file, link etc are all working cross browser.
Selenium Selenium Test your web application across a wide range of web browsers. [free, open source] (Thx Ricky L.)
Vischeck Vischeck Simulate different types of color vision and colorblindness to see what your webpage looks like to people with these conditions.
Color Filter Color Filter See what a webpage looks like to the colorblind.