Chapter 4 XML Processing
focuses on representing data rather than end user presentation. While XML aims
to separate data from presentation, the end user presentation of XML data is nev 
ertheless specifically addressed by additional XML based technologies in rich and
various ways.
Although XML documents are not primarily intended to be read by users, the
XML specification clearly states as one of its goals that  XML documents should
be human legible and reasonably clear.  This legibility characteristic contributed
to XML's adoption. XML supports both computer and human communications,
and it ensures openness, transparency, and platform independence compared to a
binary format.
A grammar along with its vocabulary (also called a schema in its generic
acception) defines the set of tags and their nesting (the tag structure) that may be
allowed or that are expected in an XML document. In addition, a schema can be
specific to a particular domain, and domain specific schemas are sometimes
referred to as markup vocabularies. The Document Type Definition (DTD)
syntax, which is part of the core XML specification, allows for the definition of
domain specific schemas and gives XML its  eXtensible  capability. Over time,
there have been an increasing number of these XML vocabularies or XML based
languages, and this extensibility is a key factor in XML's success. In particular,
XML and its vocabularies are becoming the lingua franca of business to business
(B2B) communication.
In sum, XML is a metalanguage used to define other markup languages.
While tags help to describe XML documents, they are not sufficient, even when
carefully chosen, to make a document completely self describing. Schemas
written as DTDs, or in some other schema language such as the W3C XML
Schema Definition (XSD), improve the descriptiveness of XML documents since
they may define a document's syntax or exact structure. But even with the type
systems introduced by modern schema languages, it is usually necessary to
accompany an XML schema with specification documents that describe the
domain specific semantics of the various XML tags. These specifications are
intended for application developers and others who create and process the XML
documents. Schemas are necessary for specifying and validating the structure and,
to some extent, the content of XML documents. Even so, developers must ulti 
mately build the XML schema's tag semantics into the applications that produce
and consume the documents. However, thanks to the well defined XML markup
scheme, intermediary applications such as document routers can still handle docu 
ments partially or in a generic way without knowing the complete domain specific
semantics of the documents.

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