Somehow you can get the background image of the desktop, set it your html background, and code any app in it. when you do this concept with active desktop in desktop configuration, I get to see this. ( I maximize the web page and lock it - to make it feel like my desktop )
For getting the background, I am putting the location of that in my PC right now. But I think there should be some programmatic way to do it.
This works for our local desktops. But the idea you are talking about, you definitely require Prism like thing. But Firefox looks like it stopped that project for all. (I keep the dump of it in my PC, though). So Recent users would not have prism even if you guide them to install it on their PCs.
And then, This works if the image is full sized to fit the desktop. Otherwise, We have to repeat it, and the whole desktop looks absurd. I often try to write AIR ( Adobe RIA platform for the Desktops.) apps for my taste of eyes.
I think you should try learning Adobe AIR. In fact, it supports all open technologies. I am not any Adobe employee though :) in case you think I am promoting AIR.
