I am developing a Markdown Style Guide at: http://www.cirosantilli.com/markdown-style-guide
Comments and alternatives are very much appreciated.
I started it before CommonMark was released, but I believe that it works well for CommonMark as well.
I also recommend adding a style guide to the CommonMark project: I totally understand if you think this might be out of scope, but the bottom line is: if experienced users (read: CommonMark core devs, not me ) don’t make those hard decisions, less experienced ones will have to (or worse, write without a consistent style…).
A style guide is specially important given redundancies like: "_" and "*" being synonymous is redundant and confusing present in the language which are unlikely to go away.