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1.3 HTML and XHTML: What They Are
HTML and XHTML are
document-layout and hyperlink-specification languages. They define
the syntax and placement of special, embedded directions that
aren't displayed by the browser but tell it how to
display the contents of the document, including text, images, and
other support media. The languages also tell you how to make a
document interactive through special hypertext links, which connect
your document with other documents on either your computer or
someone else's as well as with other Internet
resources.
You've certainly
heard of HTML, and perhaps XHTML too, but did you know that they are
just two of many other markup languages? Indeed, HTML is the black
sheep in the family of document markup languages. HTML was based on
SGML, the
Standard Generalized Markup Language. The powers-that-be created SGML
with the intent that it be the one and only markup metalanguage from
which all other document markup elements would be created. Everything
from hieroglyphics to HTML can be defined using SGML, negating any
need for any other markup language.
The problem with SGML is that it is so broad and all-encompassing
that mere mortals cannot use it. Using SGML effectively requires very
expensive and complex tools that are completely beyond the scope of
regular people who just want to bang out an HTML document in their
spare time. As a result, HTML adheres to some, but not all, SGML
standards,[3] eliminating many of the more esoteric
features so that it is readily useable and used.
Appendix D uses a subset of SGML to define the HTML 4.01
standard.
Besides the fact that SGML is unwieldy and not well suited to
describing the very popular HTML in a useful way, there was also a
growing need to define other HTML-like markup languages to handle
different network documents. Accordingly, the W3C defined the
Extensible Markup
Language (XML). Like SGML, XML is a separate formal markup
metalanguage that uses select features of SGML to define markup
languages. It eliminates many features of SGML that
aren't applicable to languages like HTML and
simplifies other SGML elements in order to make them easier to use
and understand.
However, HTML Version 4.01 is not XML-compliant. Hence, the W3C
offers XHTML, a reformulation of HTML that is compliant with XML.
XHTML attempts to support every last nit and feature of HTML 4.01
using the more rigid rules of XML. It generally succeeds, but it has
enough differences to make life difficult for the standards-conscious
HTML author.
