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The essence of XML is in its name: Extensible Markup Language. It lets you define your own tags, the order in which they occur, and how they should be processed or displayed.Another way to think about extensibility is to consider that XML allows all of us to extend our notion of what a document is: it can be a file that lives on a file server, or it can be a transient piece of data that flows between two computer systems (as in the case of Web Services).XML is a meta-language: a language that allows us to create or define other languages. We need it because HTML is specifically designed to describe documents for display in a Web browser, and not much else.For example, with XML we can create other languages, such as RSS, Math ML (a mathematical markup language), and even tools like XSLT. It becomes cumbersome if you want to display documents in a mobile device or do anything that’s even slightly complicated, such as translating the content from German to English.HTML’s sole purpose is to allow anyone to quickly create Web documents that can be shared with other people.XML, on the other hand, isn’t just suited to the Web – it can be used in a variety of different contexts, some of which may not have anything to do with humans interacting with content (for example, Web Services use XML to send requests and responses back and forth).And all the code used in the book is available to customers in a downloadalbe archive.

A human can certainly read this document and make the necessary semantic leaps to understand it, but a computer couldn’t. Humans are much better at semantics than computers, because humans are really good at deriving meaning.The others either roll their eyes in anticipation of hype and half-formed theories, or cringe in fear of a long, dry history of markup languages.As a result, I’ve learned to keep my explanation brief.Semantics and Other Jargon You’re going to be hearing a lot of talk about “semantics” and other linguistics terms in this chapter. For example, if I asked you to list as many names for “female animals” as you could, you’d probably start with “lioness”, “tigress”, “ewe”, “doe” and so on.If you were presented with a list of these names and asked to provide a category that contained them all, it’s likely you’d say something like “female animals.” Furthermore, if I asked you what a lioness was, you’d say, “female lion.” If I further asked you to list associated words, you might say “pride,” “hunt,” “savannah,” “Africa,” and the like.