Running layout in WASM is already practical. A good demo is https://www.nicbarker.com/clay
The things you can't do are things like expose an accessibility tree (without a dummy DOM), interact with the system IME, and access system fonts.
The things you can't do are things like expose an accessibility tree (without a dummy DOM), interact with the system IME, and access system fonts.
Even input you might be able to hack around but accessibility is a big deal and the "hack" at that point is nearly to both lay it out in the browser and the supposed "fixed" layout system, and while that may work in some sense I again have lots of questions about whether that is really "fixed".
It's solvable, except for what isn't solvable because the browser doesn't expose it, which can be solved by fixing that too. But it's a lot more work than meets the eye. And building the layout engine is hard enough in the first place. Give me a week and an AI budget and I'll produce you some sort of layout engine that works when fed exactly the sorts of inputs I anticipated, but to build something that survives contact with the real world is going to be well beyond something you just prompt your way around today.
FWIW, I actually think it would be much more valuable just to fix the spec and make CSS layout fast-by-default.