An API can return JSON, XML, HTML microformats, maybe even a binary encoding of some kind, possibly more down the road. All of these are serialized formats for

An API can return JSON, XML, HTML microformats, maybe even a binary encoding of some kind, possibly more down the road. All of these are serialized formats for objects. Of course you can have a unique URL for each format and avoid the accept header, just like you don't technically need more than GET and POST methods.