The reason that no browser does this is because that's the easiest way to go from "some sites are broken in this browser because no webdevs care to test with it" to "many sites are broken in this browser because webdevs deliberately go out of their way to break it". It's the nuclear option that a browser like Firefox would only go for (in the climate of the current web) if things got truly desperate for them. It's why, for example, Firefox didn't start blocking third-party cookies until Safari did it first: it gave them political cover to do so (and Safari can do whatever it wants because it's the only game in town on iOS).