I am requesting (via Accept: application/json) that an API I'm designing respond as JSON. However, I want the values within that JSON to be specified to conform to text/plain or text/html depending on the capabilities of the client.

What is the RESTful best practice for a "sub-type"? How would this work if I formally switched to HAL as the top-level container?

Accept: application/json+text/plain

{
  "value": "Hello World"
}

Accept: application/json+text/html

{
  "value": "<h2>Hello World</h2>"
}