HTML and CSS Reference
[*] An alternative to DTDs is XML Schemas. Schemas offer features related to data typing and are more
programmatically oriented than document-oriented. For more information, check out XML Schema by
Eric van der Vlist (O'Reilly).
Even with stylesheets, there are limitations to presenting XML-based in-
formation. Let's say you want to create something more challenging,
such as a DTD for musical notation or silicon chip design. While describ-
ing these data types in a DTD is possible, displaying this information
graphically is certainly beyond the capabilities of any stylesheets we've
seen yet; properly displaying this type of graphically rich information
would require a specialized rendering tool.
Nonetheless, your recipe DTD is a great tool for capturing and sharing
recipes. As we'll see later in this chapter, XML isn't simply about creating
markup languages for displaying content in browsers. It has great prom-
ise for sharing and managing information so that those precious kum-
quat dishes will be preserved for many generations to come. Just bear in
mind that, in addition to writing a DTD to describe your new XML-based
markup language, in most cases you will want to supplement the DTD
[ ] In fact, it is possible to write XML documents using only a stylesheet. DTDs are highly recommen-
ded but optional. See http://www.w3c.org/TR/xml-stylesheet for details.
15.1.2. A Little History
To complete your education into the whys and wherefores of markup
languages, it helps to know how all these markup languages came to
In the beginning, there was SGML. SGML was intended to be the only
metalanguage from which all markup languages would derive. With
SGML, you can define everything from hieroglyphics to HTML, negating
the need for any other metalanguage.
The problem with SGML is that it is so broad and all-encompassing that
mere mortals cannot use it. Using SGML effectively requires very ex-
pensive and complex tools that are completely beyond the scope of reg-