If CSS selector behaviour and the DOM elude you—especially if you use them in production and they still elude you—this is a must watch: it's relatively short[1], no nonsense, and though not definitive, it's a nice way to get up to speed with what's behind HTML5 and why all the buzz.

[1]: Definitely shorter than what I'd suggest as an alternative—Mark Pilgrim's history of HTML5 <http://diveintohtml5.info/past.html>.

(PS - @DanielRibeiro: what a small world, a friend of mine works at the company you used to be CTO of!)