Idk man, I’ve used a lot of UI toolkits, and I don’t really see anything wrong with GTK (though they do basically rewrite it from scratch every few years it seems…)
The only thing that comes to mind is the React-ish world of UI systems, where model-view-controller patterns are more obvious to use. I.e. a concept of state where the UI automatically re-renders based on the data backing it
But generally, GTK is a joy, and imo the world of HTML has long been trying to catch up to it. It’s only kinda recently that we got flexbox, and that was always how GTK layouts were. The tooling, design guidelines, and visual editors have been great for a long time