I have to say I find the EPUB specs absolutely hilarious. They started with an overly complicated and still utterly inadequate spec in EPUB 2.0. They made it even more complicated and still utterly inadequate in EPUB 3.0 and then they rolled back the metadata spec to something somewhat, but not quite similar to what it was in EPUB 2.0 for EPUB 3.1.
And just to put icing on the cake EPUB 2.0 metadata is incomatible with EPUB 3.0 metadata which is incompatible with EPUB 3.1 metadata and EPUB 3.1 metadata while superficially similar to EPUB 2.0 metadata is incompatible with it as well.
Does anyone take these people seriously?