REST prefers JSON

Web services are meant to be language-neutral which facilitate communication between disparate systems. REST APIs combined with JSON as the data interchange format is becoming the default choice for building web services. These APIs are published over the HTTP protocol and get invoked by passing some headers and a message body. Unlike SOAP which requires XML as its body, RESTful APIs don't enforce a constraint on the format. This flexible approach allows for the message body types to be Text, XML, HTML, binary data, or JSON, which is a text format easily readable by humans and machines. JSON doesn't have complex data types, but instead defines only a handful of them that are supported by almost every language. This makes JSON a very appealing choice as a data-interchange format for web services.

According to http://json.org:

JavaScript Object Notation...