Web Design in a Nutshell, 3rd Edition

XML Documents

XML documents may be used for a wide variety of content. A document might be text based (such as a magazine article), or it might contain only numerical data to be transferred from one database or application to another. An XML document might also contain an abstract structure, such as a particular vector graphic shape (as in SVG) or a mathematical equation (as in MathML).

A Brief XML History

Both XML and HTML have roots in SGML (Standard Generalized Markup Language). SGML is a comprehensive set of syntax rules for marking up documents and data that has existed as an ISO standard since 1986. It is the big kahuna of meta-languages. For information on SGML, including its history, see http://www.oasis-open.org/cover/general.html.

When Tim Berners-Lee needed a markup language that told browsers how to display content, he used SGML to create HTML. In other words, HTML is an SGML application, albeit a very simplified one.

As the Web matured, there was a clear need for more versatile markup languages. SGML provided a good model, but it was too vast and complex; it had many features that were redundant, overly complicated, or simply weren’t useful. XML is a simplified and reduced form of SGML.

Much of the credit for XML’s creation can be attributed to Jon Bosak of Sun Microsystems, Inc., who started the W3C working group responsible for scaling down SGML to its portable, Web-friendly form. Other big players include James Clark, the technical lead of the working group, and Tim Bray, ...