Imagine something like "base system" with WebAssembly, bare bones HTML engine with canvas support and no CSS. So now you can just download your favourite JS engine, your favourite HTML, CSS engines. But browser does not necessarily need to sheep those. It would allow to lower the bar for entry into browser market. I think that even a single person could implement that kind of base browser.

Of course there're many hard questions. For example downloading an entire JS+HTML+CSS engine with every website is not a good approach, some caching must be done. There should not be version hell with every website asking for a different version of engine. But those questions could be approached as a more general web library issues.