i found a (imho) very good book 'bout html5. Its calles 'DIVE INTO HTML 5'.
It takes care of very wide range of html5 features and feature-detection.
There is an HTML-Vesion hostet at http://diveinto.html5doctor.com
On chapter 2 you will find intresting facts about the creation of the all known <img> tag, from a time before w3c was founded. (Att al not very different from some Discussions about new wb-features).