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 details of the errors are seen when looking the HTML source of the page.
The extension is based on Tidy. Tidy, was originally developed by the Web Consortium W3C. And now extended and improved by a lot of persons. Tidy is embedded inside Mozilla/Firefox and makes the validation locally on your machine, without sending HTML to a third party server.
Tidy is a helpful program that tries to help people to correct their HTML errors. It finds HTML errors and classifies them in 3 categories:
errors: HTML errors that Tidy cannot fix or understand.
warnings: HTML errors that Tidy can fix automatically
(optional) accessibility warnings: HTML warnings for the 3 priority levels defined in W3c WAI
If you have only Warnings, press on the „Clean up“ button and Tidy will do his best to propose you a cleaned version of your page.