For the record, I learnt it the hard way while debugging/optimizing an animated fixed-layout eBook a few months ago.
One particular page was crashing iBooks on an iPad Mini 2 Retina while it was OK on an iPad 2 (non-Retina).
Turns out iBooks on 1st Gen Retina devices doesn’t really like animations in which dozens of spans are animated independently.
It took days to refactor and optimize the script + the CSS so that it just doesn’t crash iBooks (you can witness a visible framerate drop at the end of the animation, shame on me). If we could have used canvas, we would have gone full canvas—but we couldn’t because font import in canvas is problematic in iBooks.
Weeks after that nightmare, we discovered several books in the iBookStore were crashing iBooks too because there were too many interactions in a XHTML file (animations + autoplay audio via JS for instance)… The publishers weren’t able to reproduce the problem because they didn’t own the devices impacted.