Simpler HTTP APIs
A quick note on writing HTTP APIs in an RPC style ... also sharing my enthusiasm for RDF and RDF schemas all around :-)
Typical HTTP API design
Someone put together a crazy big decision diagram explaining which status code to return under which circumstance, etc.
In practice, many HTTP service APIs work more like a Remote Procedure Call and less like a fully conforming "Hypermedia Service". Headers are usually considered "low level", used for things like caching, ETags, cookies, CORS, etc.