@jonny depending on how things go, i might end up writing one for myself if i decide i want to use html as a canonical content format which gets converted into other serializations. i'm not particularly enamored with markdown as a canonical content format (although i recognize why many people like it) so i don't know how much energy to devote to markdown extensions if i won't end up using them
but i still have research to do re: content modeling and management...