Is it really a standard if the dominant vendor just ignores the standard and does what it wants because then it becomes the "actual standard" ?
As a (terrible) example, look at FIPS. The government has the power to mandate a standard that everyone needs to implement. If instead of "supporting specific browser vendors" they "support a specific standard" then all vendors have a target that works with an agreed upon common ground.
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.
Basically, no proprietary features whatsoever. No draft features. No optionally-supported standard features without a fallback that fully covers the same underlying use case.
When people complain that they can't submit their DMV forms (or whatever) and you say, "well we followed all the standards, go find another browser" and they say "which one" and you say "I don't know, we didn't test with any browsers, we just test against the standard" who do you think they'll blame?