Every API MUST support HTTP/1.1 and MUST adhere to its semantic.
HTTP Protocol Quick Start
The understanding of HTTP starts with the understanding of HTTP message and its routing.
Once you are familiar with the HTTP message structure learn about the HTTP request methods, HTTP response status codes and HTTP headers.
Each HTTP request method, status code, and header have its semantics defined, and every API MUST strictly adhere to it.
Follow the Robustness Principle. Use only the HTTP request methods, response codes and HTTP headers you understand, be liberal in accepting others.
The following documents are great overview of the HTTP protocol and related standards:
Alternatively, you can download HTTP cheat sheets at HTTP posters.
The HTTP protocol semantics is defined in the following RFCs: