Baker Richard - Learning XML: The Ultimate Beginner’s Guide to Learn XML Programming Step by Step

Figure 1.1 - An HTML page viewed in a browser

There is a close relationship between HTML and XML; both are based on SGML (Standard Generalized Markup Language). As the name implies, SGML is a general markup language with enormous possibilities, flexible and powerful and therefore can be very difficult. XML is a subset of SGML, but easier to use (note that, technically speaking, HTML is considered an application of SGML). For more information on the relationship between SGML and XML, see the document www.w3.org/TR/NOTE-sgml-xml .

When thinking of markup to specify how the content of a document can be handled, it is easy to see that many types of markup languages already exist. For example, if you use a word processor to save a document in RTF (Rich Text Format), you will find a large variety of markup codes included in the document. Here is an example where an RTF file was created with Microsoft Word with the letters "abc" underlined in bold. Try searching for the actual text (it's near the end):

The currently most used markup language is HTML, but it is easy to see that this language is only suitable for creating standard web pages.

HTML 1.0 is made up of about a dozen tags, while the most recent version, HTML 4.01, is made up of nearly 100 tags, and if you include the other tags added by major browsers, the number approaches 120. However, as it spreads of managing data on the web, it is clear that 120 tags are not enough.

For example, what if you have a hobby of building model ships and want to exchange data with other users? HTML does not include tags such as , , , or any other necessary. What should you do if you work for a very large bank and want to exchange financial data with other institutions, you will prefer tags like ,

and or tags like , and < TRANSFERACCOUNT> ? (In fact, some markup languages already exist, including Extensible Business Reporting Language, made with XML.)


Likewise, what should a web browser manufacturer do who wants to create a specialized markup language to allow people to configure the browser, add scrollbars, toolbars, and other elements? It will have to create its own markup language. Netscape did this with a user interface language called XML-based User Interface Language, which we will discuss in this chapter.

The bottom line is that there are so many reasons to create markup languages and numerous methods to manage data and of course both are unlimited. Here comes XML, which allows you to create custom markup languages.

What does XML look like?

What does XML look like and how does it work? Here is an example that mimics the HTML page just shown:

xml version = "1.0" encoding = "UTF-8"?>

Hello From XML

Welcome to the wild and woolly world of XML.

We will see in detail the parts of an XML document in the next chapter, but in this chapter we will have an overview of how it works: we will start with the XML processing instruction xml version = "1.0" encoding = "UTF-8"?> (all XML processing instructions begin with and end with ?> ), indicating that you are using XML version 1.0, l only version currently defined, and the UTF-8 character encoding, a stripped down 8-bit version of Unicode; details on this will be found later in the chapter. Also, as you add new sections of code, they will be highlighted with shadows to underline the scanned lines.