I like the ideas behind this but I do not think this will take off as much as CommonMark has. My main reasons for thinking this:
1. Users don't really care about edge cases. CommonMark (cmark-gfm really) is "good enough". If something looks odd people go "oh, strange" and make a 3 second edit and forget about it.
2. There's already multiple parsers for every language for markdown. Here there is a single Lua implementation.
3. GitHub is the king.
If it cannot render in GitHub it's not real. If it's harder for me to write in the 99% case then it's not worth it. If it's harder for me to implement within my application then I'm too lazy to switch.
Nice things in here:
1. Less ambiguous parsing. For the parser infrastructure I support at $JOB this makes supporting questions from users a HUGE pain in the ass.
2. AST-first design. Seems like everything appears in the AST (even references!) which is a huge win over cmark-gfm. There's also source positions included which is a MASSIVE win for tooling.
3. Custom Attributes implementation. This is really nice for extensibility. It is less readable but having it as an option is nice.