my ePubs have an image cover page with structure borrowed from a baen book. Flight Crew reports that the svg element can't sit in one of several tags. It's actually in a body element which is not mentioned in the list.
So either it's not a problem (and it does seem to work) or the error message needs tweaking!
All of my epubs seem to pass apart from that. Mind you, none of them will open properly in Stanza so either there is more to ePub than we know or stanza's more picky...
Thanks for making this!