The Extensible Markup Language is considered the de facto standard for delivery of structured data on the Web. A subset of SGML, it is a markup language that is used primarily to describe data in a way similar to how HTML is used to describe the visual structure of Web documents.
The terminology used to describe XML documents is defined in the body of the XML 1.0 specification, available from the World Wide Web Consortium (W3C) Web site at www.w3.org/TR/REC-xml. A relevant subset has been paraphrased here:
XML declaration— This declaration denotes the beginning of an XML document. It specifies the version of XML specification being used, and optionally, the document's encoding. Examples:
<?xml version="1.0"?> <?xml ...