There are a bunch of “small subset of HTML” UI toolkits already. The first that comes to mind is Sciter: https://sciter.com/ which bundles QuickJS interpreter, built in support for React-like syntax, and a HTML/CSS like markup and rendering environment. You’ll see on the homepage how it’s used in many products you might have heard of.

The issue here is re-use - Sciter is small and very fast, but won’t run an arbitrary existing web app that targets Chrome. Maybe you could argue for a middler-ground, add more HTML features to Sciter until it can run “most” things… but you’ll end up back to having the whole banana.

That's a really interesting idea, but in practice you might find that the "conservative sunset" ends up being different for every toolkit and developer.

Maybe something a bit opinionated, like a React Native-only runtime, could fit the bill here.

Or in a sense, is this what Flutter is?