The sum language of HTML+JS+CSS, or in other words html files. I haven't seen a better way to write UI in text files, other UI frameworks relies a lot on building the UI in some special editor or aren't easy to use.
It's not geared, imho, for a UI with lots of interactivity - e.g., imagine trying to implement blender or an audio processing app (like audacity) in html/css. You'd basically be creating new primitives on top of html, and build your UI on that (or install one of those UI libraries that _renders_ to html).