What I like in EPUB3 is the CSS pseudo-element support, which allows to shorten the code or some functions that EPUB2.* does not support, for example
letrine
https://developer.mozilla.org/en-US/...seudo-elements
p::first-letter {
font-size: 1.5rem;
}
etc
As for the development of PDF, as it fell right at the beginning, the source of the offense was secured as an internal launch of malicious scripts.