The issue with this is that the browser is the cross-playing operating system, the VM that runs webapps. But we treat the platform like an evolving document format. If we want to declare it complete, we need to make it extensible so we can have a stable core without freezing capabilities. I foresee all of this CSS/HTML stuff as eventually being declared a sort of legacy format and adding a standard way to ship pluggable rendering engines/language runtimes. WASM is one step in that direction. There are custom rendering/layout engines now, but they basically have to render to canvas and lose a lot of performance and platform integration. Proper official support for such engines with hooks into accessibility features and the like could close that gap. Of course, then you have every website shipping a while OS userland for every pageload, kinda like containers on servers, but that overhead could probably be mitigated with some caching of tagged dependencies. Then you have unscrupulous types who might use load timings to detect cache state for user profiling... I'm sure there's a better solution for that than just disabling cross-site caching...

I digress.