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Site Accessibility Tester Add-on for Firefox

This is a work-in-progress add-on for Firefox to serve as an aid to developers ensuring their web content is accessible.

While it's in the initial stages of development, it will only be available as a Temporary Add-on.


  • Check out the repository locally
  • From the root folder of the project, run the following:
  • sass popup/scss/choose_test.scss popup/css/choose_test.css --style compressed to generate the CSS
  • tsc to compile the Typescript into JS
  • Open Firefox and point to about:debugging#addons in the address bar
  • Click on the Load Temporary Add-on... button and navigate to the project directory and load the manifest.json file there

Using the Add-on

Note: This add-on will only be available for the current session that it's loaded due to it being in a temporary load state.

Once the add-on has been loaded, visit any page, and open the Firefox developer tools (F12 on most systems). This will show a menu similar to the following:

Accessibility add-on main menu

Tests are grouped under the Layout, Visual, or Audible tabs, depeding on the type of test. Each one will either affect the current page being viewed (such as one of the colour blindness filters) or it will output an error to the browsers console window (if there is any error to report).

For example, running the "Heading Levels" test on the test sources/heading fail test.html page of the project will give an error report in the console similar to this:

Accessibility add-on example error table

The colour blindness and blur filter tests affect the page currently being viewed, and have no error output in the console. These tests are to help simulate the page under particular visual conditions, so that you can more easily identify any problem areas. Running the blur and achromatopsia colour blindness tests on my How Readable is Your Content? blog post will show something similar to this:

Blur and achromatopsia simulation filter being applied to a web page

Error Messages

If a test produces an error message table, it will list out each element that fails the test. For example, if the heading levels on a page don't have a logical order, the entire heading list will be output.

Beside each element is a small icon:

Firefox DOM inspect icon

Hovering this will highlight the element on the page, and clicking it will take you directly through to that element in the DOM inspector. This will help you get to the source of any problems more quickly.

The Tests

Section Test Name Description
Layout Heading levels Check that the page starts with an <h1> level heading, at the levels are logical throughout the page, and don't skip levels for visual purposes
Layout Form element labels Check that all visible form elements have labels that either wrap the element, or are associated via an id and for combination, and ensure that labels are not set to display: none; or visibility: hidden;
Layout Bold tags Look for any use of <b> tags
Layout Italic tags Look for any use of <i> tags
Layout Table appearance Checks table elements have correct role attributes if CSS changes their display property to a non-table type
Layout Event handlers Looks for elements not interactive by default for any inline event handlers (e.g. onclick) and looks for an interactive role value assigned to them
Layout Tabindex Checks for elements with positive tabindex values, which cause issues for people relying on assistive tech
Visual Image descriptions Check that all images have alt text, or are within <figure> elements that contain a <figcaption>. This will also throw a warning if large images are found with empty alt text, which could indicate a development error
Visual Colour contrast Look at all text and check the colour contrast ratio is at least 3:1 for large text or 4.5:1 for regular text, as detailed in the Web AIM Colour Contrast Checker
Visual Font size Checks for text below the minimum threshold of 14px
Visual Toggle CSS Toggle all CSS (found in external stylesheets, <style> blocks, or inline style attributes) on or off on the page
Visual Video captions Ensure that <video> tags contain at least one set of captions of the type captions, subtitles, or descriptions
Visual Autoplay video Check for any <video> elements with the autoplay attribute set
Visual Low vision (blur) Applies a blur filter to the page to simulate poor vision
Visual Colour blindness (all) Applies a colour blindness simulation filter to the page. Only one colour blindness filter can be applied at any one time
Visual Reset filters Remove any blur or colour blindness filters currently applied
Audible Audio captions Ensure that any <audio> tags contain at least one set of captions of the type captions, subtitles, or descriptions
Audible Autoplay audio Check for any <audio> elements with the autoplay attribute set

Planned Updates

  • Improved error output and better messaging when there's no error to show
  • Publishing this as a full plugin
  • More tests (obviously)


Plugin for Fx to test the accessibility of a web page




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