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16.1 Why XHTML?
HTML, as everyone should know by now, began as a simple markup
language similar in appearance and usage to other SGML-based markup
languages. In its early years, little effort was put into making HTML
perfectly SGML-compliant. As a result, odd features and a lax
attitude toward enforcing the rules became standard parts of both
HTML and the browsers that processed HTML documents.
As the Web grew from an experiment into an industry, the desire for a
standard version of HTML led to the creation of several official
versions, culminating most recently with Version 4.01. As HTML has
stabilized into this latest version, browsers have become more alike
in their support of various HTML features. In general, the world of
HTML has settled into a familiar set of constructs and usage rules.
Unfortunately,
HTML offers only a limited set of
document-creation primitives, is incapable of handling nontraditional
content such as chemical formulae, musical notation, or mathematical
expressions, and fails to adequately support alternative display
media such as handheld computers or intelligent cellular phones. We
need new ways to deliver information that can be parsed, processed,
displayed, sliced, and diced by the many different communication
technologies that have emerged since the Web sparked the digital
communication revolution a decade ago.
Rather than trying to rein in another herd of maverick, nonstandard
markup languages, the W3C introduced XML as a standard
way to create new markup languages. XML is the framework upon which
organizations can develop their own markup languages to suit the
needs of their users. XML is an updated version of SGML, streamlined
and enhanced for today's dynamic systems. And while
the W3C originally intended it as a tool to create document markup
languages, XML is also becoming quite useful as a standard way to
define small languages that different applications use as
data-exchange protocols.
Of course, we don't want to abandon the plethora of
documents already marked up with HTML, or the infrastructure of
knowledge, tools, and technologies that currently support HTML and
the Web. Yet, we do not want to miss the opportunities of XML,
either. XHTML is the bridge. It uses the features of XML to define a
markup language that is nearly identical to standard HTML 4.01 and
gets us all started down the XML road.
16.1.1 XHTML Document Type Definitions
HTML 4.01 comes in three variants, each defined by a separate SGML
DTD. XHTML also comes in three variants, with XML
DTDs corresponding to the three SGML DTDs that define HTML 4.01. To
create an XHTML document, you must choose one of these DTDs and then
create a document that uses that DTD's elements and
rules.
The first XHTML DTD corresponds to the
"strict" HTML DTD. The strict
definition excludes all
deprecated elements (tags and attributes)
in HTML 4.01 and forces authors to use only those features that are
fully supported in HTML. Many of the HTML elements and attributes
dealing with presentation and appearance, such as the
<font> tag and the align
attribute, are missing from the strict XHTML DTD and have been
replaced by the equivalent properties in the CSS model.
Most HTML authors find the strict XHTML DTD too restrictive, since
many of the deprecated elements and attributes are still in
widespread use throughout the Web. More importantly, the popular
browsers while fully supporting the deprecated
elements have yet to fully implement the new standard ones. The
only real advantage of using the strict XHTML DTD is that compliant
documents are guaranteed to be fully supported in future versions of
XHTML.[2]
[2] If the W3C has its way, HTML
won't change beyond Version 4.01. No more HTML; all
new developments will be in XHTML and many other XML-based
languages.
Most authors will probably choose to use the
"transitional" XHTML DTD.
It's closest to the current HTML standard and
includes all those wonderful, but deprecated, features that make life
as an HTML author easier. With the transitional XHTML DTD, you can
ease into the XML family while staying current with the browser
industry.
The third DTD is for frames. It is identical to the transitional DTD
in all other respects; the only difference is the replacement of the
document body with appropriate frame elements. You might think that,
for completeness's sake, there would be strict and
transitional frame DTDs, but the W3C decided that if you use frames,
you might as well use all the deprecated elements as well.
