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Validate at W3C
Validate at W3C - Contextual menu item in Opera to submit the current page to the W3C's HTML validator...
W3C Tool Kit
W3C Tool Kit - Fast access to W3C Markup and CSS Validator...
W3C (X)HTML Validation
A Firefox add-on that provides you quick access to a W3C markup validation service...
W3C HTML Batch Validator
A free Python script that will call the W3C HTML Validator in batch mode...
W3C Offline Page validator
Firefox add-on that allows you to validate pages using W3C Markup validation service....
Chiba
An open source Java implementation of the W3C XForms standard...
Zorba
Free and open source XQuery processor...
process processor implement W3C implementation implement W3C