> there's nothing in the spec that prohibits extensions to the set of fields
Correct. But since it has all this "JSON-LD" stuff baked in, you need to also carefully define and publish the extra fields.
Correct. But since it has all this "JSON-LD" stuff baked in, you need to also carefully define and publish the extra fields.
Of course, you run the risk that you end up with de-facto standards and everyone having to implement one implementation's extensions (oh, hi WebKit-prefixed properties!).