I never got the sense that markdown was even designed, per se - the original markdown.pl script, so far as I understand it, was not intended to implement a form

I never got the sense that markdown was even designed, per se - the original markdown.pl script, so far as I understand it, was not intended to implement a formal language at all; its attitude was pure descriptivism, simply rendering what the author thought of as "plain text" in a more typographically-pretty format.