Well, that was the plan, and it was largely even true for EPUB 1 and EPUB 2.
EPUB 3 wound up being so baroque and overblown that there's still not even one fully-compliant implementation, more than 5 years after the standard was published. They had to back off some time ago and bless efforts that implemented only subsets of the full spec, but the damage had been done.
It's the second-system effect from hell (even though it was technically the third system).
