Earlier we compared formatting payloads in JSON versus XML. One of the chief advantages of sticking with XML is that it can be validated, that is, that disparate systems can independently confirm that a given XML document is well-formed and complies with a particular schema. Schemas can be thought of as contracts that define the elements an XML document should contain and what constraints the elements and their values should be held to. Document Type Definitions (DTDs) are an older schema format that's losing favor among developers because of its lack of support of namespaces. Additionally, its successor, the XML Schema Definition (XSD), is well-formed XML and uses a far richer datatyping system.
Another of the chief advantages of sticking with XML is that most server-side languages will support it without needing any additional libraries, possibly without any additional processing at all. In fact, if the source data is already in some kind of XML-based language, another XML-based language known as Extensible Stylesheet Language Transformations (XSLTs) is probably all that will be needed. XSLT facilitates the translation of one flavor of XML to another.
