This is CMark with a fairly bare bones wrapper around it.
There exist pure Zig markdown parsers, so why bother doing this? The only up-to-date Zig markdown parser is actually a port of the github-flavored-markdown fork of CMark, which on paper does not appear to implement the 0.30 version of the commonmark spec (despite that version being over 2 years old at this point). I also do not need or want the GFM extensions, which are poorly documented. Ironically, one of the interesting features (footnotes) is completely omitted from the "gfm commonmark spec", as far as I can tell. Additionally, the pure Zig port is not dependency-free. It uses libPCRE to run the regular expressions required by the parser, whereas commonmark has them pre-compiled to C source with re2c.
Plus I wanted to have a markdown parser project with an Asciidoc readme.