What?? Has somebody already done it? Darn...
My feeble claim to usefulness for this is that you can stick it in your epub file, and not require any modifications to your browser, and it can be nice to have for visually inspecting an epub during development. And it's rather fun messing about with CSS3 and javascript. I'm still fiddling about with it, and have got it working in Chrome and Opera too. Trying to get javascript unzipping and html5 file reading to work as well.