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CSS Feature Toggle Extension

This devtools extension provides the ability to toggle-off CSS features, allowing developers to see how their pages and applications render and fallback in browsers that don't support modern CSS features.

Available for Chrome via the Chrome Web Store.

Screengrab of the CSS Feature Toggle extension

Supported browsers

This extension uses devtools API features that aren't supported in Firefox yet:

  • devtools.inspectedWindow.getResources()
  • devtools.inspectedWindow.onResourceAdded
  • resource.getContent()
  • resource.setContent()


Toggling CSS features isn't supported everywhere yet so you should be aware of these caveats:

  • Inline styles (<div style="...">) will not be disabled.
  • Changes made to a stylesheet via the CSSOM will not be disabled.


Installing for development

  1. Clone this repo.
  2. Start Chrome, open the Extensions manager and enable Developer Mode.
  3. Click Load unpacked extension and select the repo folder (the one containing manifest.json)
  4. Open devtools and click the CSS Features tab to use the extension.

Making code changes

  1. Update code
  2. Close/re-open devtools to see your changes


In the /tests/browser directory you'll find a test page which can be opened in a supported browser. The page contains one or more tests for each toggle. If you make changes and want to submit a pull request, please ensure the tests behave first. If you're adding a new toggle, please try to update the page with relevant tests or ask for assistance in your pull request.

How it works

The extension uses browser.devtools.inpectedWindow.getResources() to fetch the content of stylesheet and document resources loaded in the current window. For stylesheet resources, resource.getContent() is used to extract the style content. For document resources, a script is injected into the document which extracts the CSS content from <style> elements.

Extracted style content is passed through a series of regular expressions that rename the feature property/value/identifiers. The original content is then replaced with the modified CSS using either resource.setContent, for stylesheets or - for document styles - by injecteding a script to update the relevant <style> elements. For example:

  • display: flex becomes display: -disabled-flex
  • @supports (...) {} becomes @-disabled-supports (...) {}
  • transform: ... becomes @-disabled-transform: ... {}
  • width: calc(...) becomes width: -disabled-calc(...)


A devtools extension for toggling CSS features allowing developers to see how pages/apps render in browsers that don't support modern CSS features





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