[KevinH]

Originally Posted by TheTechAuthor

Apple's own documentation points to this web page: https://www.w3.org/TR/html401/struct/text.html and it was from here I got the idea for using <em> instead (and that just works).

That is actually a very very old html 4.01 spec that is not part of epub3 which actually calls for the xhtml serialization of html5 not 4.

You should be using an html5 based validator and not that old spec. It actually violates the epub3 spec in more than a few places.