> I'm deeply familiar with React
I'm not trying to be snarky here but... React is not declarative.
Though it's good you highlighted that as that is essentially the fundamental difference here: WPF/QT are declarative, CSS is declarative, UIKit is (mostly?) declarative, HTML is declarative(-ish). React components aren't.
It's a far cry from an immediate mode API where you are making draw calls.
And it's not really representing UI: that's the main point here. Layout is decoupled; it's only really representing front-end logic and state.
I liked this about react, you create a render function and render your stuff. It is miles of head or the angular horrible architecture but the Web needs basic components implemented naively that developers can extend and not start every time from Zero with divs and other basic stuff.
However, what's less excusable is thinking React components are somehow equivalent to XML: React components are a lot more than bits of JSX (components very often don't contain any JSX, it's an optional syntax extension, not a fundamental part).
There's very little in common between React components and the XML definitions used by Qt,
> UI with an XML syntax language
If you're interested in learning a bit about how React works, I'll start you off with this: React doesn't really do UI directly at all. It isn't layout-aware, layout is handled by the rendering target: e.g. DOM+CSS for the ReactDOM rendering lib, HTML+CSS for the ReactDOMServer rendering lib, UIKit for the RN iOS rendering lib, etc.