Epub3 specs are available, although I gather not definitive.
It lets the author do a lot of things (most of them useless IMHO) and relaxes the syntax - which would create huge problems to the hardware developers (not computer, they can always ask for another 16GB RAM or 23GHz trillion-core processors - but small devices have a limited HW).
I dare to recall the java experience. Java is a sort of C++, but without strict syntax and stuff. A heaven for lazy programmers, that don't need to care about memory, heap, stacks and so on - the system does them for him. No wonder everyone could program in java, and the result was sloooow, inefficient and memory-hungry programs, that crashed much often than their pendants, written by real programmers in real languages. And it's not even portable, many modifications must be made to port an alleged portable software from Linux to Windows, or vice versa (the things got a bit better today, as there were written cross-libraries, yet another layer).
Look at the internet today. Nothing can work if javascript is disabled. Flash applets and other complicated stuff make possible to navigate within 3 pages of the site
. The internet page weights now a median of 1MB, with all javascripts and flash up to 10MB. When all these could have been done in less than 10kB pure HTML. So if one has a free quota of 300MB then pay for each MB (and surfs at reduced speed ie modem-speeds) he'd understand this "better". 1kB useful information, 9999kB ballast.
My advice would be always, keep it simple.