Actually, as a decoder implementer, I found that having the C source code of a reference decoder (libvpx), whose behaviour is authoritative over any text document, actually made things a lot more easy.
Because now, whenever you ask yourself &quot;how should a VP9 decoder behave in this situation?&quot;, you have a non-ambiguous answer &quot;do like libvpx does&quot;.<p>We didn&#x27;t have this luxury with HEVC. The HEVC reference decoder (HM) is horribly complex, and has been easy to crash in many different ways for years. As a consequence, it could by no means be considered as authoritative. Which is a pity, because there were a lot of  discrepancies between the reference decoder and the .doc spec.