HTML and CSS Reference
ular people who just want to bang out an HTML document in their spare
time. As a result, developers created other markup languages that are
greatly reduced in scope and are much easier to use. The HTML stand-
ards themselves were initially defined using a subset of SGML that elim-
inated many of its more esoteric features. The DTD in Appendix D uses
this subset of SGML to define the HTML 4.01 standard.
Recognizing that SGML was too unwieldy to describe HTML in a useful
way and that there was a growing need to define other HTML-like
markup languages, the W3C defined XML. XML is a formal markup
metalanguage that uses select features of SGML to define markup lan-
guages in a style similar to that of HTML. It eliminates many SGML ele-
ments that aren't applicable to languages such as HTML, and simplifies
other elements to make them easier to use and understand.
XML is a middle ground between SGML and HTML, a useful tool for de-
fining a wide variety of markup languages. XML is becoming increas-
ingly important as the Web extends beyond browsers and moves into
the realm of direct data interchange among people, computers, and dis-
parate systems. A small number of people wind up creating new markup
languages with XML, and many more people want to be able to under-
stand XML DTDs in order to use all of these new markup languages.