Well, it took about a decade for web standards to become a real thing and a lot longer for Web Platform Tests to come to be. Still, while there are lots of tests for DOM construction and visual rendering, testing construction of the accessibility tree is lacking (also keyboard interaction testing).

And that's just for browsers, there's no shared spec for the operating system accessibility APIs the browsers' accessibility tree has to be translated into or how screen readers (and other assistive technologies) will use the OS's APIs.