The most extreme version of this was a colleague who wrote a parser for the "human readable" version of the H265 spec. This could then be turned into executable code - and also a set of test streams.
On a much smaller scale, VPRI made part of their TCP stack by parsing ASCII diagrams of packet contents (e.g. see the STEPS reports at http://vpri.org/writings.php )
Edit: Ah, I see user nradov has already mentioned this in a sibling comment :)
Wow, that is some truly impressive piece of technology! Kudos to those who successfully completed the daunting task of actually implementing this pretty crazy idea.
This resulted in him filing a large number of minor bugs between the spec and the reference implementation: https://hevc.hhi.fraunhofer.de/trac/hevc/query?status=accept...
And a product: http://www.argondesign.com/products/argon-streams-hevc/ (page includes explanatory video)