I've recently implemented an AVIF encoder (built on ISO HEIF), and working with the specs was quite unpleasant. It's a tower of ISO standards built on top of other ISO standards, with each level they're getting older, broader, and less relevant to the original format they were supposed to be for, so sum of all these specs makes something that is bloated and overcomplicated.

A key spec in this pile was paywalled, so I've reverse-engineered an existing implementation instead. And guess what? The implementation I used as a reference wasn't compliant with the paywalled standard either. So now AVIF-in-the-wild is not compatible with the ISO-AVIF. Great job, ISO!