Originally Posted by Toxaris
That is not saying anything... I suspect you viewed it with iBooks. Apple took a shortcut and used WebKit as rendering engine for books with some adaptions. However, an ePUB is a subset of xhtml and css, not the full set. So, some things will work that will not work on anything else because it is not in the specs. Also, their 'adaptations' messed up some things badly by not following the specs.
Your results may be quite different compared to your aim.
Most on this forum advise to test at least on ADE. That seems to be the safest choice at the moment.
Apple is targeting iBooks toward ePub 3 not the current standard used by the vast majority of eBook readers, ePub 2. ePub 3 has more web like features in it.