There's a Firefox/Chrome extension called Axe that's pretty good. We use at my company it to make our products accessible and it helped us tremendously.
The engine behind that add-on, Axe core, can be called from JS and there are some open source tools around to integrate it in your CI. I would say Axe is kind of the gold standard at this time when it comes to automated accessibility testing. Not because it catches the highest number of issues, but if it flags something you can be pretty sure it's a real issue and not a false positive.
[1] https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/
[2] https://chrome.google.com/webstore/detail/axe-web-accessibil...