HTML5 is indeed far away from replacing Flash or Silverlight UI technologies. And the problem is not only that HTML5 is limited in amount of required in an app UI components, but also that it doesn't allow create custom ones in such a way that they would integrate into runtime seamlessly.
Take a look at Ample SDK - JavaScript UI Framework, it's architecture largely inherit from what you will find in Flex, Silverlight or Gecko. One of the UI technologies it enables cross-browser is XUL, styleable in CSS and programmable in JavaScript with XUL DOM or jQuery APIs. You can also create your own UI technology on top of the framework.
