29 June 2005

What is XML?

(Part 2)

For those of you wondering what XML is, but too embarrassed to ask, XML stands for Extensible Markup Language. It is a meta-language, which means it is a formula for defining languages that employ tags to define the appearance of document elements enclosed in those tags. A really common markup language is HTML; but XML entered common usage several years after HTML in order to correct several problems with HTML's "looseness," or tolerance for sloppy usage. As a consequence, HTML is not an implementation of XML; XHTML, or extended HTML, is.

What this specifically means is that XHTML complies with the guidelines for defining an XML-compatible language. As such, XHTML is known as an XML application, and is one of literally hundreds of applications. Others include MathML, used for exchanging mathematical formulae online; CML, for Chemistry Markup Language; X3D, the current version of the virtual reality markup language (VRML); RSS (for syndicated feeds of blogs or news sites); SVG (for scalable vector graphics); and something known as XFDL (extensible forms description language), which is required for sharing data among e-commerce sites.

XML was derived as a dialect of SGML (a “subset”) that could be delivered over the Internet. It has a parallel history to HTML; HTML 3.2/4.0 were compatible with XML 1.0 []. However, the two are not substitutes. While HTML tells a browser how to display data, XML merely describes the data. XML was created to structure, store and to send information.

W3 Schools XML Tutorial: The tags used to mark up HTML documents and the structure of HTML documents are predefined. The author of HTML documents can only use tags that are defined in the HTML standard... XML allows the author to define his own tags and his own document structure. ...Tags are "invented" by the author of the XML document.

W3 Schools XML Tutorial: When HTML is used to display data, the data is stored inside your HTML. With XML, data can be stored in separate XML files. This way you can concentrate on using HTML for data layout and display, and be sure that changes in the underlying data will not require any changes to your HTML.

XML data can also be stored inside HTML pages as "Data Islands". You can still concentrate on using HTML only for formatting and displaying the data.
XML is an official standard maintained by the World Wide Web Consortium (W3C), an international foundation founded by Tim Berners-Lee.

Since HTML is a language with a defined vocabulary, and XML has no defined vocabulary—that's the job of the application—all web pages created with XML applications absolutely positively must have a stylesheet that specifies what the terms actually mean. There are currently two rival formats for stylesheet languages—CSS and XSL. The latest versions of both are compatible with XML, and while XSL is specifically designed for use with XML, CSS2 is compatible with HTML (XSL can generate, but not read, HTML documents [* via *]).

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