The .epub file itself is just a zip archive. You can open it that way and pull the HTML content out, if you want, although it may be a pain to work with absent some kind of conversion - I think Calibre can linearize an epub into a single HTML file, but I haven't actually needed to do that so can't say for sure.
That said, it's not the worst standard, if a little bit overcomplicated by trying to be all things to all people with a bunch of features I suspect are almost never actually used in the wild. Having the content already formatted as XHTML is really convenient for what I'm doing with it, too.
