The problem is browser vendors wouldn't want to implement that spec because if updating your browser breaks the website, no matter how much you explain it to the user, it's your fault, not the website owner's. It's why we have Quirks Mode even after 15 years. It's why Linus is so adamant about patches breaking userspace;[1] if your update broke it, it's your fault, no matter how bad the truly broken thing is.
[1]: https://lkml.org/lkml/2012/12/23/75