| The Extensible
Markup Language (XML) describes a class
of data objects called XML documents
which are stored on computers, and partially describes
the behavior of programs that process these objects.
XML is a subset or restricted
form of SGML, the Standard Generalized Markup
Language (ISO 8879). The goal of XML
is to enable generic SGML to be served, received,
and processed on the Web in the way that is now
possible with HTML. XML has been
designed for ease of implementation and for interoperability
with both SGML and HTML.
XML
customizes SGML in a number of significant ways.
- First, a specific
choice of syntax characters was made so that
everyone using XML will use the same concrete
syntax. For example all start tags must begin
with "<" and end with ">".
- Second, a new empty-element
tag may be used to indicate that this is an
empty element and that an end tag is not expected.
This new empty-element tag is like a start
tag with a slash character just before the
closing greater-than angle bracket.
- Third, tag omission
is not allowed as it is in SGML. This means
that each non-empty element will have a both
a start tag and an end tag.
- Finally XML does
not require that a Document Type Definition
be present.
XML
was developed by the SGML Editorial Board formed
under the auspices of the World Wide Web Consortium
(W3C) beginning in 1996. The design goals for
XML are:
- XML
shall be straightforwardly usable over the
Internet.
- XML
shall support a wide variety of applications.
- XML
shall be compatible with SGML.
- It shall be easy
to write programs which process XML
documents.
- The number of optional
features in XML is to be
kept to the absolute minimum, ideally zero.
- XML
documents should be human-legible and reasonably
clear.
- The XML
design should be prepared quickly.
- The design of XML
shall be formal and concise.
- XML
documents shall be easy to create.
- Terseness in XML
markup is of minimal importance.
Today XML is a W3C
Recommendation. This means that XML has been
reviewed and approved by the members of the
W3C. XML is therefore deemed "stable" and ready
for widespread deployment. A Recommendation
is the highest level a W3C document can be assigned.
XML became a Recommendation on February 10,
1998.
You can find the XML
specification by linking to http://www.w3.org/TR/1998/REC-xml-19980210. There are several versions of the specification
including a .htm (html version) and a .pdf (that
can be read with Acrobat).
Useful
XML Web Links:
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