The problem with targeting a subset is there's a ratchet effect with web APIs, once support reaches critical mass in the major browsers sites will start unconditionally relying on those features and there's no going back from there, any new browser has to also support those features or be considered broken.
I suppose anything that's gated behind a permission prompt in Chrome/Firefox/Safari could be culled without too much trouble at least.
There is a reason that Esperanto or one of it's siblings is not the language of global understanding and we are discussing in English. The world at large generally does not care about these kinds of fancy. It's half a miracle that we agreed to whatever the Chromium engine implements to be a baseline that most folks build stuff on.
I think people forget the early 2000s. Many sites only worked on IE but Firefox was the better browser. Firefox users had an extension that would open certain links in IE or open the current page in IE via the context menu.
If a lightweight browser could be significantly faster and more secure, people would tolerate using two browsers again. Although Ladybird hasn't reached that bar.
YouTube certainly could use a small set of web standards, although YT regularly breaks on Firefox. It's a video player with links and forms.
I suppose anything that's gated behind a permission prompt in Chrome/Firefox/Safari could be culled without too much trouble at least.