Is it possible to destroy EVERYTHING on a given page when playing the game from http://erkie.github.com?
No, it's not always possible. Usually some items stick around with a 0×0 hitbox thus preventing you from bringing the page down to the bare background.
You'll know you've gone as far as you can when holding the B button shows no blinking targets.
One such site, for example, is the (classic) Twitter profile page.
Taken from the source code:
Thus ignoredTypes are not valid enemies