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This section only describes the rules for XML resources. Rules for
text/html resources are discussed in the section above entitled "The HTML
syntax".
The XML syntax for HTML was formerly referred to as "XHTML", but this specification does not use that term (among other reasons, because no such term is used for the HTML syntaxes of MathML and SVG).
The syntax for XML is defined in XML and Namespaces in XML. [XML] [XMLNS]
This specification does not define any syntax-level requirements beyond those defined for XML proper.
XML documents may contain a DOCTYPE if desired, but this is not required
to conform to this specification. This specification does not define a public or system
identifier, nor provide a formal DTD.
According to XML, XML processors are not guaranteed to process
the external DTD subset referenced in the DOCTYPE. This means, for example, that using entity references for characters in XML documents
is unsafe if they are defined in an external file (except for <,
>, &,
", and ').