18 The XHTML syntax
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 the XML specification, 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 XHTML documents
is unsafe if they are defined in an external file (except for