I'm going to post this one without much comment.
![A screenshot of modernizr running in iBooks](/img/epubtest1.jpg)
You can find more information on Modernizr on its website and if you want to, you can download the "Modernizr in ePub" file itself.
ETA (July 2012): Apple turned off access to many of these APIs a while back. The lesson is that iBooks isn't a stable development environment.