8.10. Outgoing Representations

Most of the documents you serve will be representations of resources, but some of them will be error conditions. Use HTTP status codes to convey how the client should regard the document you serve. If there’s an error, you should set the status code to indicate an appropriate error condition, possibly 400 (“Bad Request”). Otherwise, the client might treat your error message as a representation of the resource it requested.

The status code says what the document is for. The Content-Type response header says what format the document is in. Without this header, your clients won’t know how to parse or handle the documents you serve.