Earlier quoted context omitted.
With all major browser JIT compiling JavaScript, aren't they already?
No, because it still is the pile of HTML, CSS and JavaScript hacks. With WebAssembly you can bypass all of it, and do your UI framework in GL, and everything else with native libraries compiled into WebAssembly.
Seriously, DON'T do this. This breaks ctrl-f. This is unlikely to work well for people who need to enlarge text or enhance contrast due to vision impairment. This breaks screen readers. This will probably break most site archive navigators, so your content is lost to history (e.g. wayback machine). This will probably prevent Google from indexing your site, killing your page ranking and driving away potential customers. Even if you re-implement all the things you think it will break, you'll find that your users have things set up in ways you'd never considered...
Just, don't. Please don't do that.