What is a11y.css used for?

a11y.css is a CSS stylesheet and browser extension that highlight accessibility and markup mistakes in HTML pages, for quick visual auditing by developers. It is a good fit for quickly auditing page markup for accessibility issues and checking image alternatives during development.

Is a11y.css open source?

Yes. a11y.css is open source under the MIT licence. Its source code is on GitHub at ffoodd/a11y.css.

Yes. a11y.css is open source, so the software itself is free to use.

What are some alternatives to a11y.css?

Similar open-source tools in the Testing & QA category include Pa11y CI, Playwright and k6. SaaS products in the same category include Acceslab, BrowserStack and Chromatic.

Is a11y.css actively maintained?

The most recent commit to a11y.css was on 24 April 2026, and the latest release is v5.3.0, published on 10 December 2024. The project has 1.6k stars on GitHub.

a11y.css: open-source testing & QA

About a11y.css

a11y.css is a CSS file that warns developers about possible risks and mistakes in HTML code. By including it as an external stylesheet, you can roughly evaluate a site's quality, because problematic markup such as missing alternatives or bad attributes is visually flagged on the page. It is aimed at front-end developers and accessibility reviewers who want a fast check without running a full audit tool.

The stylesheets are available in several languages, including French, English, Spanish, Greek, Arabic, Portuguese, Russian, Chinese, Polish and Dutch, and in leveled or full versions. A WebExtension for Firefox, Edge and Chrome, kept in its own repository, adds features that the plain CSS lacks, such as forcing focus visibility, showing lang attributes, displaying image alternatives and increasing text spacing. It can also toggle the CSS, change the level on the fly, remember settings per tab and update itself in the background.

The project is MIT licensed and is written in SCSS. Its wiki documents the checks and explains how to get involved, and issues and pull requests are welcome. It is a diagnostic aid rather than a certification tool, so it complements manual accessibility testing.

Key features

Visual warnings for risky HTML markup
Leveled and full stylesheet versions
Available in ten languages
Browser extension for Firefox, Edge and Chrome
Forces focus visibility and shows lang attributes
Per-tab settings and background updates

Good fit for

→Quickly auditing page markup for accessibility issues
→Checking image alternatives during development

Built with: SCSS
Tags: accessibility; a11y; css; html; audit; browser-extension; web-development; scss

a11y.css: questions and answers

What is a11y.css used for?: a11y.css is a CSS stylesheet and browser extension that highlight accessibility and markup mistakes in HTML pages, for quick visual auditing by developers. It is a good fit for quickly auditing page markup for accessibility issues and checking image alternatives during development.
Is a11y.css open source?: Yes. a11y.css is open source under the MIT licence. Its source code is on GitHub at ffoodd/a11y.css.
Is a11y.css free?: Yes. a11y.css is open source, so the software itself is free to use.
What are some alternatives to a11y.css?: Similar open-source tools in the Testing & QA category include Pa11y CI, Playwright and k6. SaaS products in the same category include Acceslab, BrowserStack and Chromatic.
Is a11y.css actively maintained?: The most recent commit to a11y.css was on 24 April 2026, and the latest release is v5.3.0, published on 10 December 2024. The project has 1.6k stars on GitHub.