Firefox logo

This page describes an easy way to validate HTML documents using CSS HTML Validator and Firefox.

CSS HTML Validator is a desktop software application for Windows 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: Easy Copy Add-on (Recommended)

The Firefox Easy Copy add-on extension by Byron (new window) 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.

Note: Unfortunately this add-on is not currently compatible with Firefox Quantum.


  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=" ">Success

  4. Run Firefox and install the Easy Copy add-on extension by Byron (new window). Use Firefox with the add-on to copy HTML to the clipboard. See the sections below for important information and tips.



Option 2: Html Validator Add-on with CSS HTML Validator Support

information item Unfortunately this add-on has problems with current versions of Firefox and CSS HTML Validator and no longer appears to be under development. Consider using the integrated web browser in CSS HTML Validator instead. It allows you to browse the web while validating the HTML source.

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