My definition of compatible is basically what I described in the comment: "[not being] the cause of 2/3rds of the lines of exceptional UI code".
There's the occasional situation where Chromium doesn't support something or Firefox doesn't support something, but it feels like Safari has double the rate of both combined.
The computing industry has a pretty poor track record of multiple-implementations ever agreeing or being a net positive for the user. Instead it always seems to devolve into pile-of-hacks for platform X and pile-of-hacks for platform Y, regardless of what the specification says.
What is the definition of “compatible” here?
The point of “one specification, multiple implementations” is to explicitly leave the door open for slightly deviant implementations.