Originally Posted by Jellby
That's one of the problems with CSS in ePUB, there's no way to refer to the actual display page/screen size (height or width). The containing block is usually <body>, and it takes the width of the page (so width references typically work as expected), but its height is unspecified and adapts to its contents. Could you consider specifying your image's height in ems? It will change when changing the font, but that may be good too.
I am having another problem with font over a background-image i am going to ask in MR: EPUB format