This isn't really how it works though. It READS epub3, but only data that are valid epub2 structures is rendered. To get a subset (not even all) of epub3 format

This isn't really how it works though. It READS epub3, but only data that are valid epub2 structures is rendered. To get a subset (not even all) of epub3 formatting it needs to be built as a Kepub. You may be able to trick it with the extension, but there is some syntax difference. It really is a conversion if you want anything approaching true epub3 and even then its as mentioned a subset.