But we can still have websites that work against existing browsers while also being strictly conforming to the standard, surely?
Basically, no proprietary features whatsoever. No draft features. No optionally-supported standard features without a fallback that fully covers the same underlying use case.
Plenty of standards are guidelines. The point is the website’s purpose trumps dogmatic adherence to a standard. If the site works against standards but not in the browser, it’s a failure.