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I'm surprised this is well defined behavior.

It certainly is now (HTML5 retroactively standardized a lot of quirks), but it wasn't always well-defined: https://scrappy-do.blogspot.com/2004/08/little-rant-about-mi...

I already saw the Stack Overflow reference cited in the article (https://stackoverflow.com/questions/8318911) many years ago, so I wasn't surprised.

But I guess I can still be disappointed. Postel's Law has its limits.

Postel or Hyrum?

Postel. This is about me thinking it was a bad idea to make it work in the first place.

But reason people are making it compatible with old versions of browsers is part of Hyrum's law.