> text/plain is a superset of everything that can be represented by plain text

Not in the context of web forms.

Just checked the spec and "text/plain" just seems to be an alias for "application/x-www-form-urlencoded" [1] - i.e. stuff that looks like

  key=value&anotherkey=anothervalue

on the wire.

Apparently though, keys and values can contain arbitrary characters and arent percent-encoded, so you can do a "quine" where the "key" is

  {"foo": "bar", "ignore": "

and the "value" is

  "}

And then the browser will happily send

  {"foo": "bar", "ignore": "="}

over the wire, which is valid json.

[1] https://html.spec.whatwg.org/multipage/form-control-infrastr...