What changed in CommonMark?

Semi-formal, yes (I’ll make a post about this in a bit).

Basically, I think implementations should write their own standards, written in the form of amendments to the core standard, that document their divergences with as much rigor and clarity as the main standard. This way, other implementations can copy the divergences of others (chjj/marked having an option to support all the features of GitHub Flavored Markdown, as an example), and even file bugs against the original implementation, pointing out where it diverges from the personal standard it describes.