"No javascript" is a non starter in my opinion. That's a very simple on/off switch that is already available but has very little buy in. As you noted, "JS off" mode requires a shift in what HTML/CSS are capable of on their own.
> Making the standard is easy, it is getting anyone to follow it that is difficult
That's my point, those two parts aren't disconnected. The standard isn't useful (or a standard really) until people follow it, and in this case that's most of the internet connected world. Both people building for the a new default subset, and users accepting a default subset with opt in "web app" bells and whistles.
Without removing JS, in my head it's along the lines of starting with a freeze of a current ECMA version, define the API's that are stripped out, force low fidelity timers, remove JIT, limit some cross origin options. Stop adding shiny new feature's every 8 weeks. Keep it there for 3-4 years. Or maybe a similar concept with a WASM container when it gains some browser usefulness. Then there's the html and css subset too. So, defining that stock subset navigator at the right level is what I see as the "hard" part.
