HTML is a perfectly valid response, meaning the Content-Type is possibly the same as the page. Ajax can be done synchronously. DELETE and PUT and other methods are acceptable and becoming common because they are preferable and necessary for real REST. Closures are not a requirement. The data transfer is rarely JSON on the way up, where regular url-encoded params are usually used, but often so on the way back.

A shallow summary with incorrect definitions and muddy use of terms like "RPC" ingrains misimpressions.