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April 2, 1999
Web posted at: 7:02 p.m. EST (0002 GMT)

by Mark Johnson

(IDG) -- Despite HTML's popularity, it is severely limited in what it can accomplish. It's fine for disseminating informal documents, but HTML now is being used to do things it was never designed for. Trying to design heavy-duty, flexible, interoperable data systems from HTML is like trying to build an aircraft carrier with hacksaws and soldering irons: the tools (HTML and HTTP) just aren't up to the job.

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The good news is that many of the limitations of HTML have been overcome in XML, the Extensible Markup Language. XML is easily comprehensible to anyone who understands HTML, but it is much more powerful. More than just a markup language, XML is a metalanguage -- a language used to define new markup languages. With XML, you can create a language crafted specifically for your application or domain.

XML will complement, rather than replace, HTML. Whereas HTML is used for formatting and displaying data, XML represents the contextual meaning of the data.

This multi-part article will:

• Present the history of markup languages and how XML came to be.
• Look at sample data in HTML and move gradually into XML, demonstrating why it provides a superior way to represent data.
• Explore the reasons you might need to invent a custom markup language, and I'll teach you how to do it.
• Cover the basics of XML notation, and how to display XML with two different sorts of style languages.
• Dive into the Document Object Model, a powerful tool for manipulating documents as objects (or manipulating object structures as documents, depending upon how you look at it).
• Explain how to write Java programs that extract information from XML documents, with a pointer to a free program useful for experimenting with these new concepts.
• Look at an Internet company that's basing its core technology strategy on XML and Java.

The origins of markup languages

Berners-Lee defined HTML in SGML, the Standard Generalized Markup Language. SGML, like XML, is a metalanguage -- a language used for defining other languages. Each so-defined language is called an application of SGML. HTML is an application of SGML.

SGML emerged from research done primarily at IBM on text document representation in the late '60s. IBM created GML ("General Markup Language"), a predecessor language to SGML, and in 1978 the American National Standards Institute (ANSI) created its first version of SGML. The first standard was released in 1983, with the draft standard released in 1985, and the first standard was published in 1986. Interestingly enough, the first SGML standard was published using an SGML system developed by Anders Berglund at CERN, the organization that, as we have seen, gave us HTML and the Web.

SGML is widely used in large industries and governments such as in large aerospace, automotive, and telecommunications companies. SGML is used as a document standard at the United States Department of Defense and the Internal Revenue Service. (For readers outside of the US, the IRS are the tax guys.)

Albert Einstein said everything should be made as simple as possible, and no simpler. The reason SGML isn't found in more places is that it's extremely sophisticated and complex. And HTML, which you can find everywhere, is very simple; for a lot of applications, it's too simple.

HTML: All form and no substance

Admittedly, artists and hackers have been able to work miracles with the relatively dull tool called HTML. But HTML has serious drawbacks that make it a poor fit for designing flexible, powerful, evolutionary information systems. Here a few of the major complaints:

HTML isn't extensible

An extensible markup language would allow application developers to define custom tags for application-specific situations. Unless you're a 600-pound gorilla (and maybe not even then) you can't require all browser manufacturers to implement all the markup tags necessary for your application. So, you're stuck with what the big browser makers, or the W3C (World Wide Web Consortium) will let you have. What we need is a language that allows us to make up our own markup tags without having to call the browser manufacturer.

HTML is very display-centric

HTML is a fine language for display purposes, unless you require a lot of precise formatting or transformation control (in which case it stinks). HTML represents a mixture of document logical structure (titles, paragraphs, and such) with presentation tags (bold, image alignment, and so on). Since almost all of the HTML tags have to do with how to display information in a browser, HTML is useless for other common network applications -- like data replication or application services. We need a way to unify these common functions with display, so the same server used to browse data can also, for example, perform enterprise business functions and interoperate with legacy systems.

HTML isn't usually directly reusable

Creating documents in word-processors and then exporting them as HTML is somewhat automated but still requires, at the very least, some tweaking of the output in order to achieve acceptable results. If the data from which the document was produced change, the entire HTML translation needs to be redone. Web sites that show the current weather around the globe, around the clock, usually handle this automatic reformatting very well. The content and the presentation style of the document are separated, because the system designers understand that their content (the temperatures, forecasts, and so on) changes constantly. What we need is a way to specify data presentation in terms of structure, so that when data are updated, the formatting can be "reapplied" consistently and easily.

HTML only provides one 'view' of data

It's difficult to write HTML that displays the same data in different ways based on user requests. Dynamic HTML is a start, but it requires an enormous amount of scripting and isn't a general solution to this problem. (Dynamic HTML is discussed in more detail below.) What we need is a way to get all the information we may want to browse at once, and look at it in various ways on the client.

HTML has little or no semantic structure

Most Web applications would benefit from an ability to represent data by meaning rather than by layout. For example, it can be very difficult to find what you're looking for on the Internet, because there's no indication of the meaning of the data in HTML files (aside from META tags, which are usually misleading). Type red into a search engine, and you'll get links to Red Skeleton, red herring, red snapper, the red scare, Red Letter Day, and probably a page or two of "Books I've Red." HTML has no way to specify what a particular page item means. A more useful markup language would represent information in terms of its meaning. What we need is a language that tells us not how to display information, but rather, what a given block of information is so we know what to do with it.

SGML has none of these weaknesses, but in order to be general, it's hair-tearingly complex (at least in its complete form). The language used to format SGML (its "style language"), called DSSSL (Document Style Semantics and Specification Language), is extremely powerful but difficult to use. How do we get a language that's roughly as easy to use as HTML but has most of the power of SGML?

The answer is XML, of a simplified form of SGML that incorporated tried-and-true features of SGML but with reduced complexity. Part 2 this series tells the story of XML's origins, and Jon Bosak, its creator.

Mark Johnson is a C++ programmer by day and a JavaWorld columnist covering the JavaBeans technology by night. Very late night.