I don't see why you couldn't support many different markup languages in a single site. For example, on a web forum I'd love to be able to write BBcode with an s-expression syntax.

So you'd have 2 <select>s: one for semantics, one for syntax. So you could have BBcode semantics with traditional syntax, BBcode semantics with s-expression syntax, and a lot of other semantics and their corresponding available syntaxes. All those representations would simply be canonicalized into one language on the server.