"XML is short for Extended Markup Language and is a subset of
SGML, the Standard Generalized Markup Language. XML is an HTML-like
formatting language. Whereas most HTML-related formats developed in
the past adopted the 'be conservative in what you send and liberal
in what you receive' attitude, XML takes the opposite
approach--documents should be 100% compatible. This compatibility
is known as 'well-formedness' of an XML document. To this end, even
when the goal is clear, a document is rejected if it does not
follow XML specifications to the fullest extent. In terms of
practicality, this approach guarantees interoperability in the long
run. Unlike HTML, which is the standard groupname for a lot of
sub-protocols that are slightly different and not fully
interoperable with one another, the strictness of XML ensures
compatibility. XML also improves security dramatically, because
there is only one way to interpret expressions, a way on which
"DocBook is an XML Document Type Definition or DTD. It is a
subset of XML particularly suited for but not limited to the
creation of books and papers about computer hardware and software.
DocBook is well-known in the Linux community and is used by many
publishing companies and open-source development projects..."
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