I've been thinking the same thing, and started[0] experimenting[1] with some ideas. I think it be fun to make a web browser that implements a few HTML tags, flexbox and a few other CSS primitives (no animations), and no JS. Sites that are compatible with it would still work on current browsers.
It's why I'm somewhat against WASM, even though it's very cool from a technical standpoint. It makes the web even more of an operating system, where I'd like it to be less.
[0] https://anderspitman.net/19/#netcatable
[1] https://anderspitman.net/17/#curlable