The trouble with running these sorts of things as a web service is that it often means the assessment has to happen after deployment.

Obviously, it doesn't have to be live. You can deploy to test.example.com and try it out there, but I think that's too late.

Easily detectable poor accessibility should be as much of a closed gate as having the layout go all wonky and the words all in the wrong place.

Ideally, it should be runnable on your local copy as you are writing it, just the same as viewing the page in a browser.