> Rust isn't a great language for describing UIs because it is too explicit in places that doesn't need to be for coding a UI.

That's is not about just Rust ... Any domain has its own optimal or sub-optimal language describing it. Criteria of optimality is a bit subjective of course. Like: INI, TOML, YAML, etc. are better (YMMV, again) for user facing configs than JSON or XML and so on.

I think that HTML is better for defining hierarchical UI structures and CSS and CSS selectors are (arguably again) better to define UI styling.

My Sciter[1] was initially using .FRM alike inputs (VisualBasic forms, good for IDE but bad for a human) but I quickly discovered that having styles, structure and code-behind-UI in single PL makes non-manageable mess. So Sciter uses HTML, CSS and JS we all familiar with. Not 100% match of course but good enough.

[1] https://sciter.com

I love Sciter, but the curse of Discord is bloated web.

They have no shame in deploying a 300 MB hot mess bundle.