But it's absolutely terrible at describing UIs.
I know it'll never happen, but if we had a layer created specifically to deal with UI then the web could have a chance at becoming a great software platform.
The reason why it was not "pushed" is SUN edited it. It was internally competing with java, like oak that seems interesting too.
Meh. Postscript (and PDF, for that matter) do a better job describing the appearance of a document: you can place things exactly where you want them on the page. SGML does a better job describing the semantic meaning of the page (though arguably no one does it well): a <p> isn't really very descriptive.
As you mention, HTML is lousy at UIs. HTML tries to do too many things, and ends up doing all of them poorly.