It all depends on your context. If you are making a website, HTML on the mobile. If you're making an API, you can use JSON, just don't call it REST. People see

It all depends on your context. If you are making a website, HTML on the mobile. If you're making an API, you can use JSON, just don't call it REST. People seem to overlook RPC over HTTP. There is nothing wrong with that model. Look at the early RPC implementations. They look like objects with a set of wlverbs. A URI can be treated logically as an object. /users is an object. Implicit ops are the HTTP verbs. How you move between objects is probably API specific. If you are using JSON, it is API specific because JSON currently lacks linking capacity.