I think the point of standards is portability. You still have to do testing on different platforms, especially when porting the first time.

I maintain a cross browser webextension (chrome and Firefox) and I would not release it on a new platform without testing. I have already found bugs in the API implementstions on Firefox and confident I'll find more. There are also differences in implemention between just those two browsers that I have to work around.

YMMV, of course