Clearly delinating quirks from intended behaviour

Exactly that. Standards were critical for compatibility and competition in the _old_ days before: 1. Free software, 2. The Internet, 3. Test-driven development, 4. Continuous Integration.

Even in the old days standards were not so great: just look at the C/C++ mess of undefined/implementation-defined behaviors[*], standards always trailing implementations, ugly politics (looking at you W3C), etc.

[*] plenty of elaboration and examples in other comments.

But in today's era, it is now possible to have a free implementation with an extensive test-suite, fast-paced CI and everything just a click way. THAT is better than some old-fashioned, designed-by-committee standard. Some behavior is ambiguous? Just discuss it, add documentation and a new test and problem solved!

"The standard is this [proprietary] implementation" used to really suck. But now it can really rock when done properly.

This being said, if some people see value in maintaining a 2nd class implementation, then agreed: "all the more power to them". As long as they don't pretend they're equal and defer to the reference implementation when in doubt then everything is fine. I think this is more or less what happened with Java BTW - and despite Oracle!