Browsers are permissive not because it's technically superior but as a concession for the end user who still wants to be able to use a poorly built website, and they're competing with browsers who will bend over backwards to render that crappy website so that they look good and your browser looks bad.
It's not a concession you want to make unless you really have to.
Well, my point is that there's unique pressure for browsers to be permissive for practical reasons beyond Postel's law even if you were building a browser in 2025 and the whole internet reset to xhtml.
And that's because the end-user is at the mercy of, but not party to, an over the air interface between the producer and consumer that you can't verify ahead of time.
So if you're consuming a stream of supposed xhtml `<p>foo<p>bar</p>`, you have to decide if you want to screw the user for the producer's mistake for a single fuck up in the website's footer.
HTML is a nightmare that had to be reverse engineered as in, rebuilt with proper engineering standards in mind, several times. HTML and CSS are both quite horrible.
It's not a concession you want to make unless you really have to.