Asciidoctor has a spec, reads pretty similarly to markdown, and is infinitely better IMO. And it (well, AsciiDoc) predated markdown!
I think markdown won because it was specifically made with HTML output in mind, instead of arbitrary output (docbook, in the case of AsciiDoc, which is pretty much infinitely malleable).
I think markdown won because it was specifically made with HTML output in mind, instead of arbitrary output (docbook, in the case of AsciiDoc, which is pretty much infinitely malleable).