Hi,
It gets worse ... see this post:
https://www.mobileread.com/forums/sho...41&postcount=1
So it seems the epub3 spec is simply no spec at all. Amazon has its own version (KF8), Apple has its exclusive own version now "application/x-ibooks+zip", and the Adobe crowd (Kobo, Sony, etc) still has no reader out there that supports much in the way of epub3 at all.
The industry, publishers, and ebook vendors are destroying themselves and the consumers are paying the price.
I had hoped things would get better, it seems it is getting worse. More vendor lock in, more DRM, more incompatible formats.