The fact that ePub = (X)HTML is perfect, however the standard doesn't incorporate HTML5 yet. But this is not the problem.

If you look at the ePub specs you'll quickly notice that the standard is way to bloated! You have to specify all kinds of settings/resources/meta-info and crap in 100.000 different files, you have to create you're own book spine, set up structure and whatever not...

HTML5 itself is descriptive enough to specify reading systems with the right information to display it as a book. The zhook guidelines specify - based on the HTML5 Outline Algorithm - how a book should be build up. So zhook is nothing more than a friendly reminder to keep it simple, slim and stick to the web standards. The only thing zhook adds is a specification on how to add a cover image to your book.. because even though the ePub standard is bloated as I don't know what - there is no standard way to add an image as the cover of you're book.

For more info about the HTML5 Outline Algorithm: https://developer.mozilla.org/en/Sections_and_Outlines_of_an...