Using the schema here: http://jsonapi.org/faq/#is-there-a-json-schema-describing-json-api (also here: https://github.com/json-api/json-api/blob/gh-pages/schema)
And the data from the homepage here: http://jsonapi.org/
I used the validator here: http://www.jsonschemavalidator.net/
And the validation failed saying that data was the only allowed top level element. If I took away links and jsonapi then it validated OK.
I don’t have the time/need to fix it, posting here in case anyone does - or anyone runs into the same problem.
