It hides that by being incredibly restricted in what you can do with it, lest you leak side channel information. To the point that you can't do much of anything useful, much less general computation. They're finely crafted Faberge eggs that break if you sneeze near them now or discover a new way of sneezing in the next few decades, not broad tools.

So... yes, you could build a "browser" like that. It would effectively have no scripting at all though, nor could it ever introduce new semantics that send data to another site, directly or transitively. You can do some stuff with that kind of system, but it's limited enough that most people don't choose it.

Gopher exists I guess? Lynx too, though lynx supports css, and that largely can't be allowed either.