Interesting idea, but Semantic Versioning is defined in terms of API compatibility. For CommonMark, I think the much more pressing question is about versioning the specification.
For example, in topic What is the point of limiting uri schemes in autolinks? I’m proposing a change to the specification that makes it simpler and more consistant. But it changes the result of one of the specified test outputs, sort of a corner-case one at that.
Technically speaking, my proposed change would have no impact on the API of an implementation, but it does change the expected output in a slightly non backwards-compatible way. How do you apply semantic versioning to that?