It does raise the question of whether wikidata defines a content type for its json markup, that is, json markup that conforms to its SNAKs ontology? Is there a list of the content types somewhere? I looked but could not find one in ContentHandler.php. thanks - john
The content *model* Wikidata uses is the one with Statements and Snaks, described by the Wikidata data model spec. The content *format* used for serialization is JSON. The mapping from the internal model to JSON is currently not formally described, the internal JSON serialization is an implementation detail. The external JSON serialization as used by the web API is similar to, but not necessarily the same as the serialization used to represent Wikidata items internally. The JSON form used in the API is documented to some extend on the Extension:Wikibase page on mediawiki.org, but it's still subject to change.
-- daniel