Firefox add-on: HTML Validator

HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon in the status bar when browsing.  The extension can validate the HTML sent by the server or the HTML in the memory (after Ajax execution). The details of the errors are seen when looking the HTML source of the page.

HTML Validator
HTML Validator

The extension is based on Tidy and OpenSP (SGML Parser). Both algorithms were originally developed by the Web Consortium W3C. Both algorithms are embedded inside Mozilla/Firefox and makes the validation locally on your machine, without sending HTML to a third party server.

Each algorithm has advantages and inconvenients. The program will give you the choice at startup. But mostly,

  • The SGML parser is the same program than the one running behind validator.w3.org
  • Tidy has accessibility check for the 3 levels defined in WAI

The extension is translated in 17 languages.


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