> The state of HTML parsing should convince you that if you follow postel's law in one browser then every other browser has to follow it in the same way.

No. Your claim expresses a critical misunderstanding of the principle. It's desirable that a browser should be robust to support broken but still perfectly parceable HTML. Otherwise, it fails to be even useable when dealing with anything but perfectly compliant documents, which mind you means absolutely none whatsoever.

But just because a browser supports broken documents, that doesn't make them less broken. It just means that the severity of the issue is downgraded, and users of said browser have one less reason to migrate.