There are 2 repositories under markdown-flavors topic.
A bidirectional Markdown to HTML to Markdown converter written in Javascript
CommonMark/Markdown Java parser with source level AST. CommonMark 0.28, emulation of: pegdown, kramdown, markdown.pl, MultiMarkdown. With HTML to MD, MD to PDF, MD to DOCX conversion modules.
Render Markdown to HTML on any website using a md tag
Yet another markdown parser, Compliant to CommonMark specification, written in Crystal.
Showdown website and documentation