It seems to me that the multiple-background stuff is a simplistic attempt at providing a generalised system for non-interactive layers. I think that being able to specify a background in the following manner would be more flexible (and not much more complex):
HTML:background {content:url(“helloworld.html”)}
Then, you would put whatever background you want in helloworld.html, using all of the flexibility of HTML and CSS, including perhaps having backgrounds of the background. Of course, you would not allow background layers to affect the cursor, be selected or otherwise directly affect their containing document, as they’d be purely presentational.
It would need more design work.