I suppose it depends what audience will be editing it. XUL used it because that was the fashion at the time, but it wouldn't be something I'd want to hand edit. Qt used its own QWT (now dated) which was a lot easier for a human to edit, albeit a bit ad-hoc.
If it's mostly consumed and edited by a machine/LLM, xml might actually be ok. Otherwise, I think something a bit less ceremonial. Obviously there's lots of options, and I imagine you may even miss much of the tooling of XML (schemas and such).
Kind of a drive-by comment, I admit, but it just struck me as a little surprising.
Also: it's been some 3 decades since I was playing with Ada ... very cool to see it getting some GUI love!
Came here to comment on it being fascinating how xml has survived here and thrives on this front. Even in react syntax, in some sense. Natural fit?
As happy user of qt desktop framework, not sure in what sense it’s dated.
Not sure how qwt relates to this tho? https://qwt.sourceforge.io/
Making coherent ux with it still seems much easier than in qml, it has the right constraints and visual editing it is reasonably lightweight, unlike qml studio. Of course it has its flaws too but UX wise it is still hard to find an equally mature cross platform toolkit.