Button with URL doesn't pass HTML validation

When a button on a form has a URL it is wrapped in an anchor tag, this does not pass validation at validator.w3.org with the error "The element button must not appear as a descendant of the a element."

Start by locating the form button rendering path used when a button has a URL, then reproduce the generated markup and check it at validator.w3.org. Done means the URL-bearing button no longer places a button element inside an anchor and the resulting HTML passes validation.

Written by the indexing model from the issue text.