I think that to some degree the lack of certification for browsers (and ditto for Java runtimes back in the days) is what is causing this. Write-once-run-anywhere became Write-once-debug-everywhere.

Java seems to finally start to deliver on this promise though, the last few years have seen quite a bit of improvement. (Still far from perfect though, just the other day I was fighting some corporate website that insisted I install a certain version of Java in a certain browser in order to be allowed access, and it crashed in terrible ways when I finally did gain access).

Imagine that your browser simply wouldn't be allowed on the web until it conformed to a standard. Just like cars aren't allowed to go on the road if they do not conform.

This is unlikely to ever happen for various reasons and I don't think it would be a good thing but the number of broken implementations and hacks-that-became-standards is too large to even count by now.