XML: An Overview

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Thanks to the World Wide Web, HTML has taken over the world. And yet, despite its ubiquity and popularity, HTML has always had a number of serious limitations. You don't have to build Web applications for very long before you run into some of them. HTML works reasonably well for formatting informal documents, but not so well for more complex tasks . It was never intended to describe the structure of data, but business needs have caused it to be used to do just that. The fact that it's being used to do things it was never intended to do has highlighted many of its shortcomings. It has created the need for a more powerful markup languageone that's data-centric rather than display-centric, one that doesn't just know how to format data, but that can give the data contextual meaning.

XML is the answer to many of the problems with HTML and with building extensible applications in general. XML is easy for anyone who understands HTML to learn, but is overwhelmingly more powerful. XML is more than just a markup languageit's a meta-language a language that can be used to define new languages. With XML, you can create a language that's tailored to your particular application or business domain and use it to exchange data with your vendors , your trading partners , your customers, and anyone else that can speak XML.

Rather than replacing HTML, XML complements it. Beyond merely providing a means of formatting data, XML gives it contextual meaning. Once data has contextual meaning, displaying it is the easy part. But displaying it is just one of the many things you can do with the data once it has context. By correctly separating the presentation of the data from its storage and management, we open up an almost infinite number of opportunities for using the data and exchanging it with other parties.

In this chapter we'll explore the history of markup languages and how XML came into existence. We'll look at how data is presented in HTML and compare that with how XML improves on it. We'll discuss why you might want to set up your own XML dialect , and we'll explore how to go about it. We'll touch on the basics of XML notation, and how XML can be displayed through translation to HTML via XML style sheets. We'll talk about document validation using both Document Type Definitions (DTDs) and XML schemas, and we'll discuss some of the nuances of each. We'll finish up by touching on the Document Object Model (DOM) and how it's used to manipulate XML documents as objects.

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