XML is a valid option as an interchangeable data serialization format. XML provides two advantages as a data representation language over previous models: 1) It is text-based, and 2) It is position independent. The challenge is that XML carries a lot of unnecessary overhead.
In short, JSON provides all the capabilities of XML in a much more efficient and widely used format.
IETF’s page on JSON is here: http://www.ietf.org/rfc/rfc4627.txt
Another great resource is http://www.json.org/