Google develops Lighthouse, which, although an extension, I believe includes some a11y checks: https://developers.google.com/web/tools/lighthouse/
Similarly, Mozilla also promoted Webhint: https://webhint.io/ (which is cross-browser)
I'd also recommend Khan Academy's tota11y, which just works as a bookmarklet: https://khan.github.io/tota11y/
Google develops Lighthouse, which, although an extension, I believe includes some a11y checks: https://developers.google.com/web/tools/lighthouse/
Similarly, Mozilla also promoted Webhint: https://webhint.io/ (which is cross-browser)
I'd also recommend Khan Academy's tota11y, which just works as a bookmarklet: https://khan.github.io/tota11y/