Can you give an example of how Google Documents is falling short of being desktop class with their HTML/CSS based UI?
Secondly, yes Google Docs uses HTML for their UI, but it's seriously abstracted I believe as Google Closure though I may be wrong. My point is that HTML and CSS can still be used, but they need abstractions for this type of app. Canvas is just another approach to the same problem.
Edit: here's a link to a video from very early on in development. http://db.tt/2YX8gpYx