We define Lossless as CRF 0 (which is usually the best quality and is
working fine with other codecs afaict), but since WebM only allows for
CRF values between 2-32 and actually has a dedicated "lossless" mode, I
suggest using that (it produces large files though, so double-checking
would be welcome).
https://trac.ffmpeg.org/wiki/Encode/VP9#LosslessVP9
Maniphest Tasks: T74443
Differential Revision: https://developer.blender.org/D7800