Rust was designed to build core mechanisms of UI, like internals of a browser.
But as a language-behind-ui it is quite suboptimal so I think the most successful UI framework for Rust most likely will use some form of DSL.
But if DSL then inevitably we are getting question: why not HTML/CSS/JS then?
Visual styles, layout, structure, UI logic like "on click here, hide stuff there and expand section over there" is what HTML/CSS/JS was designed for.
"Render therefore unto Cesar the things which are Cesar's;"