Looking to bring your HTML skills up-to-date, now that HTML5 support is growing in modern browsers? Mark Pilgrim has a nice online book by the name of Dive Into HTML5 that covers most of the big new things. It is also available in print, published by O’Reilly, under the name HTML5: Up & Running.
The book, both the free online version and the print copy, cover things like HTML5 video, offline storage, detecting browser support of HTML5 features with JavaScript, as well as an overview of the new elements. An interesting chapter on the history of HTML is also included.
It’s a short book, not intended to teach newbies about HTML so much as to bring adepts up to speed with the latest advancements.