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.

[0] https://anderspitman.net/19/#netcatable

[1] https://anderspitman.net/17/#curlable