ePub authoring

As ePub files contain XHTML files which must be rendered by an ePub reader I
wonder how much ePub authoring differs from web authoring? Do most (all?)
ePub readers follow the specifications closely? Is it sufficient to test
ePub files in a validator? Or is the situation similar to the web where web
authors spend most of the time testing their web pages in different browsers
and looking for workarounds to make browsers render the pages properly? I
wonder as I would like to customize the XSL style sheets for Keith's ePub
converter. 

Boris