I agree that a UI is well expressed as a tree, for the sake of accessibility and other reasons. I don‘t see why there is any reason to seperate the declaration of that tree from the application code. The flutter approach is bad, but all established desktop and mobile UI frameworks provide accessibility support as good or better than HTML.
In fact, HTML is ill equipped to express capital A Applications in an accessible form and needs ARIA as a bandaid to make it work.
The application runtime knows about the UI tree by the time the app is running and can tell accessibility tools about it. Wether you write it down explicitly or not doesn‘t matter. It only matters for web "pages" which are original not something that is running at all.
In fact, HTML is ill equipped to express capital A Applications in an accessible form and needs ARIA as a bandaid to make it work.
The application runtime knows about the UI tree by the time the app is running and can tell accessibility tools about it. Wether you write it down explicitly or not doesn‘t matter. It only matters for web "pages" which are original not something that is running at all.