https://addons.update.mozilla.org/extensions/showlist.php?application=firefox&category=Developer%20Tools&numpg=10&pageid=3
It's a toolbar you can validate the current page with, disable style sheets and javascript and much more - v.cool
Most likely, the pages were checked with the W3C Validator. The Validator "checks HTML documents for conformance to W3C HTML and XHTML Recommendations and other HTML standards".
<http://validator.w3.org/>
http://www.amyarver.com/home.shtml
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