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Chapter 15. XML
HTML is a maverick. It follows the rules of formal electronic
document-markup design and implementation only loosely. HTML was born
out of the need to assemble text, graphics, and other digital content
into electronic documents that could be sent over the global
Internet. In the early days of the Web's boom, the
demand for better browsers and document servers driven by
hordes of new users with insatiable appetites for more and cooler web
pages left little time for worrying about things like standards
and practices.
Of course, without guiding standards, HTML would eventually have
devolved into Babel. That almost happened during the browser wars in
the mid- to late-1990s. Chaos is not an acceptable foundation for an
industry whose value is already measured in the trillions of dollars.
Although the standards people at the W3C managed to rein in the
maverick HTML with standard Version 4, it is still too wild for the
royal herd of markup languages.
The HTML 4.01 standard is defined using the Standardized Generalized Markup Language
(SGML). While more than adequate for formalizing HTML, SGML is far
too complex to use as a general tool for extending and enhancing
HTML. Instead, the W3C has devised a standard known as the Extensible Markup
Language, or XML. Based on the simpler features of SGML, XML is
kinder, gentler, and more flexible, well suited to guiding the birth
and orderly development of new markup languages. With XML, HTML
itself is being reborn as XHTML.
In this chapter, we cover the basics of XML, including how to read
it, how to create simple XML Document Type Definitions (DTDs), and
the ways you might use XML to enhance your use of the Internet. In
the next chapter, we explore the depths of XHTML.
You don't have to understand all about XML to write
XHTML. We think it's helpful, but if you want to cut
to the chase, feel free to skip to the next chapter. However, you may
want to take a look at some of the uses of XML covered at the end of
this chapter, starting in Section 15.8.
This chapter provides only an overview of XML. Our goal is to whet
your appetite and make you conversant in XML. It is only an overview.
For full fluency, consult Learning XML by Erik
T. Ray or XML in a Nutshell by W. Scott Means
and Elliotte Rusty Harold, both from O'Reilly.
