Khaled,
AFAIK, epup is just a subset of xhtml with a subset of css2, so IMO not a kind of output format that is very well suited for TeX (well, I hardly consider html an output format at all, the output is what the browser renders out of it).
True, but CSS uses a box model too, so it should be possible to create an "initial view" document that -- provided the render engine is properly compliant -- essentially looks the same as a generated pdf (barring special pdf commands, of course). Given the pretty rigid description of how CSS should be rendered by the w3c documentation for it, any x(ht)ml+css document is a proper format (be that for print or screen. note that there are a number of stand-alone css render engines which don't rely on browsers, but are meant for integration into reader applications devices, for instance). The difference between something like epub and pdf is that the layout in the first is mutable. For digital readers, with many different viewport sizes and aspects, that's highly desirable. For print media the epub format is, of course, nonsense. Hence the desire for parallel format generation.
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