[An Apple-sanctioned HTML5 demo site][link]. This is a good first step towards getting serious about HTML5, in the sense of actually building things.
Things of note: Most of these aren’t HTML5 per-se, and they won’t work in other browsers that don’t use webkit, due to Apple only using their vendor-prefixed CSS attributes (`-webkit`).
[link]: http://www.apple.com/html5/ “Apple – HTML5”