Originally Posted by dgatwood
The only time it might be useful to have height on the html and body tag is if you're doing a single-page fixed-layout spread, such as a dedication page with a single line centered vertically. In paged media, a height of 100% means one page height (or at least it is supposed toŚnot all renderers seem to get this right).
All fixed layout ePub is different as it was designed before ePub 3. So each engine that supports fixed layed does it it's own way.