For low hanging fruits I would recommend using Lighthouse, which is integrated into Chrome (DevTools → Audit). That's what web.dev is also using [0]. Under the hood "axe-core" is being used. Another option is pa11y [1]. All three tools offer CI integration, but you still have to do some work to fully automate it. If you're using React / JSX, there is also an eslint plugin available [2].

[0] https://web.dev/

[1] http://pa11y.org/

[2] https://github.com/evcohen/eslint-plugin-jsx-a11y