HTML as the "transport"/"dialect" to keep compatibility with the existing computational world. What I mean is from the ground up, no C, no erlang (which Im a big fan of in terms of application organisation), no Rust, no Javascript. Start from first principles, OS'es closer to uni-kernels, sane sharing/privacy policies, ability for instance to start an "executable" (already an abstraction tied to the current mode of software production, albeit a perhaps useful one) with a narrow set of scoped access. Then it only has to implement a browser/subset of it to parse HTML and it can "access" most of the relevant parts of the existing software world (internet). Ditch anything complex, if necessary make a browser that just uses flexbox, no other things, no backwards, compatibility, if you need compatibility buy a normal laptop with a normal OS, that comes with all the cruft, but design this new one from the ground up.

But I also don't understand why (without compatibility layers) a browser should be as complex, or remotely as complex, as currently, specially with all the knowledge in hindsight.

I'm not saying this is easy, doable, or whatever, but that was what I meant.