an optional declaration at the top of an XML document that specifies the version of XML and an encoding declaration

The processing instruction at the top of an XML document. It begins with ?XML, includes a version identifier, required markup declaration, and encoding identifier, and closes with ?. (The XML declaration may be case-sensitive at some point; the standard at present is unclear on this issue.)

The first line of an XML file can optionally contain the "xml" processing instruction, which is known as the XML declaration. The XML declaration can contain pseudo-attributes to indicate the XML language version, the character set, and whether the document can be used as a standalone entity. An example is the XMLdeclaration that begins every valid XML file: ?xml version="1.0" standalone="yes"