BOSA Accessibility Check is a client-side JavaScript application that checks a HTML document or source code, and detects violations of a defined coding standard. Comes with standards that cover the three conformance levels of the W3C's Web Content Accessibility Guidelines (WCAG) 2.1 and the U.S. Section 508 legislation.
Particularly https://github.com/squizlabs/HTML_CodeSniffer/issues/196 which causes an error on pretty much all of our sites (since triggers this bug.