You mean the creators/publishers don't put their ePubs through validators ? Because I think they do, and the files pass. Yet they still display differently on every system. Why is that ? Why can't they develop ONE, GOOD, reading engine and adapt it to multiple devices ? Or even use one already developed:
http://en.wikipedia.org/wiki/Gecko_%28layout_engine%29
Open source, cross platform... What are they, stupid ?