Toolbox, H5 and twentytenfive are Wordpress templates that are based on HTML5. I should take a look at them and see if I can't build my own theme on one of them, instead of deriving it from the standard theme. Since I am currently a subtheme of the standard Twentyten, Twentytenfive might be the easiest - but Toolbox could also be interesting because it is a really minimal theme that I could use as a real base.