Definition:
“XML documents SHOULD begin with an XML declaration which specifies the version of XML being used.”
Notation:
<?xml version="1.0" encoding="UTF-8" standalone="yes">
Usage:
Usage of an XML Declaration in an XML Document is optional, however the version 'attribute' is required when used.
version
Describes the version of XML used.
Allowed values for version are '1.0' and '1.1'.
encoding
Describes the encoding for the document, possible values "US-ASCII", "UTF-8", "UTF-16", "ISO-10646-UCS-2", "ISO-10646-UCS-4" or "ISO-8859-1" ...
“All XML processors MUST be able to read entities in both the UTF-8 and UTF-16 encodings.”
When no encoding information has been provided, the content should be encoded using either UTF-16 or UTF-8.
standalone
Describes whether this document can be parsed standalone and does not use an external markup declaration (for default attribute values or constants).
Allowed values for standalone are 'yes' and 'no'.