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This page describes an easy way to validate HTML documents using CSS HTML Validator and Firefox. Some options are now legacy options that no longer work but are included for reference.

CSS HTML Validator for Windows is a desktop software application that provides one-click checking of HTML, CSS, SEO, spelling, accessibility, mobility (Android/iOS), JavaScript, and links.

See also: Validating a DOM with Google Chrome.

Option 1: Set Firefox's View Source to use CSS HTML Validator

This option allows you to use Firefox's View Source command to open the source in CSS HTML Validator's Editor. Note that this will not let you check the source generated from a live DOM.

  1. Open Firefox and navigate to about:config
  2. Set view_source.editor.external to true
  3. Set view_source.editor.path to (adjust the path for your version/system): C:\Program Files\HTMLValidator230\cmdlineprocessor.exe
  4. Set view_source.editor.args to: -o

Option 2: Use Chrome instead

Unfortunately, the Firefox-based options below are currently outdated and do not work with current versions of Firefox. We have kept the outdated options below in case they are still useful for any reason and for reference.

If you find another solution then please let us know. For now, see Validating a DOM with Google Chrome.

(No Longer Available) Option 3: Easy Copy Add-on

important informationNote: Unfortunately this add-on is not currently compatible with Firefox Quantum and is no longer available.

The Firefox Easy Copy add-on extension by Byron (no longer available) can be used to quickly copy the HTML source of a web page (generated from the current state of the DOM) to the clipboard by selecting its Easy Copy > HTML Source menu option. When the document source is copied to the clipboard, CSS HTML Validator can automatically detect it, paste the clipboard text into a new document, and validate it.


  1. First, download and install CSS HTML Validator for Windows (a trial version will work) and Firefox for desktop (new window) if you haven't already.
  2. Run CSS HTML Validator and then go to the Editor Options (Ctrl+F3) and select the Clipboard options page. Enable the option to Watch the clipboard for HTML documents. You may also want to enable other options like the Auto validate documents from the clipboard option.

    Screenshot of the Clipboard options in the Editor Options dialog box

  3. TEST IT (v17+): If CSS HTML Validator is running and the option to watch the clipboard is checked, then CSS HTML Validator should automatically open a new document with the line "Success" when you copy the below line to the clipboard.

    <cseclip url="http://test.domain.com ">Success

  4. Run Firefox and install the Easy Copy add-on extension by Byron (no longer available). Use Firefox with the add-on to copy HTML to the clipboard. See the sections below for important information and tips.



(No Longer Supported) Option 4: Html Validator Add-on with CSS HTML Validator Support

important informationNote: Unfortunately Firefox's current framework now makes it difficult, if not impossible, to call native programs. Because of this, this add-on no longer supports CSS HTML Validator.

The Firefox Html Validator add-on extension by Marc Gueury (new window) supports recent versions of CSS HTML Validator (v10.02 or greater, any edition). In the Html Validator add-on's options dialog, choose CSE HTML Validator for the Algorithm (highlighted in the screen shot). View the page source to see the validation results.

Select CSE HTML Validator for the Algorithm in the Options dialog