Comment by wizzwizz4

This would simply not be possible, if basic web browsers were more popular. The React / Next.js app wouldn't run, and then the users would complain, and the Frontend People would try to say "just use the WHATWG VM", and then the Enterprise People would say "we've blocked that so people can't play video games at work: fix your end" (ignoring that they only blocked programs named chrome.exe, and everyone's got Firefox or a renamed Chrome installed), and then the Frontend People would have to fix it because that's what the client demands. (For B2B, at least, but B2B is lucrative enough that best-practices proliferate.)

It's a chicken-and-egg problem. The harmful consequences of Postel's principle are well-known, but the solutions are also well-known. We only need to get past a certain threshold before the problem's effectively solved, and that can be done by starting in a particular domain and working our way outwards.