They do when there is a certification process. That’s the key to a well defined standard, a certification. The test suite is the real standard.

My personal experience with certified implementations is that they still tend to be not especially standardised. A test suite is just as susceptible to subversion as a written standard is. If that wasn’t true then unit tests is all we’d ever need to merge a PR.

Businesses still need their stuff to work on Chrome whether or not it's certified, so this is kind of a moot point.