HTML isn't a UI description. It's a document description. HTML + CSS + [browser behavior|Javascript] is the UI description, and arguably some more complex web apps are getting to the point were they've gone past the point where HTML, CSS, and JS are the best formats for describing the way they work.
They never were the best way. Other application GUI systems are less painful because they were designed for that from the beginning. HTML remains OK for documents, though.
