We're talking about performance here, not implementation complexity. Besides, it's not a win in terms of complexity if sites ship a limited subset of the Web stack to run on top of the full implementation of the Web stack that's already there.
> But that's kind of my point - if you can control which parts of the tree are exposed and which mutations are valid, you might not need to implement restyling at all. (Or in reduced scope)
Sure, you can improve performance by removing useful features. But I think it'll be a hard sell to front-end developers. Qt and GTK+ didn't add style sheets and restyling for no reason. They added those features because developers demanded them.
My point is that writing custom UI renderers using canvas and WASM might become a reasonable thing to do. For that you don't need to stick to the web stack at all, you can invent whatever language, API and data model fits your needs. Those can be a lot simpler than the DOM and therefore easier to implement with good performance.
I'm not paying for it, the DOM implementation is - with increased complexity. (E.g., HTML parsing suddenly becomes a time-critical operation because some wiseguy decided to implement animations for his website using setTimeout and innerHTML.)
And they can't drop it because a lot of sites rely on it - however, if you wrote a new, limited-purpose renderer on top of WASM, you could decide to drop it and simplify the implementation without losing much utility.
> And if you reinvent restyling, it'll be a ton of work to do better
But that's kind of my point - if you can control which parts of the tree are exposed and which mutations are valid, you might not need to implement restyling at all. (Or in reduced scope)
I'm not talking about cascading in general, but about how you can make arbitrary changes to the DOM after initial load, which the restyler has to fully support.