[zadokshi]

I am a bit confused by the question. Why wouldn’t all commonly used defacto standards be supported? Or are you talking about obscure standards which no one uses?

Any browser that doesn’t display normal websites normally will never achieve mainstream usage. Who willingly handicaps their software?

[alberth]

There’s websites decided to pointing out which standards various browsers do & don’t support.

https://caniuse.com/

(An older not as relevant one is http://acid3.acidtests.org/ )