Trend Micro shares benefits and considerations for this new web standard in a 3 part series that I found to be informative … The final part was issued today

HTML5 Security – The Good, Bad, and Ugly
http://blog.trendmicro.com/html5-thegood
http://blog.trendmicro.com/html5-the-bad
http://blog.trendmicro.com/html5-the-ugly

QUOTE: Welcome back to the final part of our miniseries on HTML5 and the security issues surrounding it. Today, we are going to look at what, in my opinion, is the scariest security concern HTML5 introduces by a long margin: BITB (Botnets In The Browser)