HTML VALIDATOR (based on Tidy and OpenSP)

navbarHTML Validator is a browser extension that adds HTML validation inside Firefox and Chrome. 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 and OpenSP. Both algorithms were originally developed by the Web Consortium W3C. And now extended and improved by a lot of persons. Both algorithms are embedded inside Mozilla/Firefox and makes the validation locally on your machine, without sending  HTML to a third party server.
  1. HTML 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

  2. OpenSP, is SGML parser, is a  professional algorithm. It is the same program running inside And now, you can embed it in Firefox !
There is also an automatic "Clean up" button that will do his best to propose you a cleaned version of your page, without errors.


Download: Firefox: Addons
Download: Chrome: extensions

Firefox old XUL extension do not work anymore with Firefox 57.
HTML Validator 0.98 is the first Web Extension version. It is a complete rewrite of the code.
The extension works now differently:
  • There is a new tab in Developer tools: Html Validator.
  • The validation happens only when the "Developer Tools" is open.
  • The extension uses a new version of Tidy 5 compiled from C to Javascript. It works now on all platforms.
  • It works in Chrome
Some features from the 0.97 are still missing. See here for more details.


Download : version 0.975 beta channel - FOR FIREFOX 40 and later  - Windows 32bits, 64 bits, Linux 32 bits, 64 bits and Mac OS X

New in Version 0.975 beta, Lot of fixes to get compatible with Firefox 48 and Electrolysis. See Release Notes.
        New Beta Channel: it is easy to install the beta version !
New in Version 0.965, Several fixes for compatibility with Firefox 44
New in Version 0.964, One single file for all platform, added Windows 64 bits support  See Release Notes.
Feedback is welcome.


Download : version - FOR FIREFOX 8.0 ONLY - Windows only (for now)

Since Firefox 4+ and rapid releases, XPCOM has been unfrozen. (see here)
This means practically that every extension by binary, like this one, needs to be recompiled for each Firefox version.
Due that Firefox version will follow one after the other, the current model of the extension is not viable anymore at long term.
I will need to change the structure deeply in a next version. Help is welcome.


Download : version - FOR FIREFOX 7.0 ONLY - Windows only (for now)


Download : version - FOR FIREFOX 6.0 ONLY - Windows only (for now)


Download : version - FOR FIREFOX 5.0 ONLY - Windows only (for now)


Download : version - Most platforms available

New features:
- Due to Numerous changes due to Firefox 4.0, the extension 0.9 is compatible only with Firefox 4.0
- The browser icon is in the add-on bar. The status bar does not exist in FF 4.0 anymore.
- This means too that all platforms binaries will need to be recompiled. Look the source and the forum if you want to help on the migration.
- To enable HTML 5 validation, it is possible to validate page with W3c Online too.

FIREFOX 3.6 and below

Download : version - Release notes. (roadmap, user guide, translationforums)

New features:
- Works with Firefox 3.6
- Works with Seamonkey 2.0
- Fix of warning in sp_start.html for some translations
- more...


If you want to see some images, click on one of the both images below.

source source<
Firefox + HTML Validator / tooltip in the browser statusbar Firefox  + HTML Validator / Page source


The extension has 3 main features:

1. Browser:
  • The HTML validation is done when browsing. The result is shown on the form of an icon in the status bar.
  • If the page contains HTML frames or iframes, you can see the validation result of the main page and the frames.
2. View Source:
  • The detail of the validation can be seen when looking the source of the page.
  • Tidy validates the HTML code and displays the result like a compiler for a programming language.
  • The validation is done based on the source and not by making a second request to the Web Server.
  • The screen is splitted in 3 parts. Above the source of the HTML, and below, the list of the errors and the explanation of the error.
  • The explanation of the errors are a personal effort to make HTML specification understandable. Because I must admit, that not all HTML errors are easy to understand. For example,

generates an error for </p>. And without an explanation about inline and block tags in HTML, from far point of view, this looks like correct HTML.

3. Cleanup:
  • It is also possible to ask HTML Validator to propose a corrected version of the page in HTML or convert it to XHTML.
  • You can compare the cleanup version and the original version in 2 tabs. (source and preview)


In comparison with web HTML validators like,
  • the data is not sent to a 3rd party server
      - So, there is no problem with firewall to validate dynamically generated HTML that are in intranets. What you can not do with web validators, except by saving the file and upload it.
      - You have also not the problem of people not daring to validate their pages by using other people machines.
  • the validation is done automatically and just by looking the page.
  • it is easy to validate dynamic page. Dynamic pages are only available in the browser and you validate when you see them.
  • it is a gui. So, the validation is faster and the interface has more possibilities.
  • Just press CTRL+U (page view) and all yours errors are there.It is done based on the page source and without accessing a second time the web server to get the HTML source.
  • If it is your website you try to correct, just press CTRL+R in the view source to revalidate the result after correcting the errors.


The extension includes:
  • Some XUL and javascripts files for the screens and presentation of the results.
  • Some HTML pages for the help on the HTML errors and warnings
  • A dynamic C library, .DLL or .SO, that contains:
    - The original Tidy source code
    - The original OpenSP source code
    - Some C code to glue make tidy XPCOM compliant and give the ability to call Tidy from Javascript and XUL.


If you are interested in helping or have ideas to improve the extension, do not hesitate.
See the roadmap.


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For any issue, please contact me <Marc Gueury>