Is there a simple / open / widely used format for representing forms?

I'm looking for a simple text based format that's used to represent forms (ie. collections of fields, with names, labels, types and possibly validation constraints).

Obviously HTML5 has some form elements but mixes in other kinds of markup.

Ideally I'm looking for the equivalent of Markdown : simple, human-readable (and writable), plain-text and widely recognised / adopted.

If there's nothing like this, I'd settle for something like XML or JSON that has at least some momentum of use between different applications / vendors.

Any emerging standards in this area?