The key to interoperability is standardization. The reason why independent resources on the Web are able to interoperate today is because the Web has standardized on:
- Addressing and naming resources -> URI
- Generic resource interface -> HTTP GET, POST, PUT, DELETE
- Resource representations -> HTML, XML, GIF, JPEG, etc
- Media types -> MIME types (text/html, text/plain, etc)
These are the standards that REST advocates.