HTML and CSS Reference
In-Depth Information
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
16.1.1. XHTML Document Type Definitions
HTML 4.01 comes in three variants, each defined by a separate SGML
Document Type Definition (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
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 re-
placed by the equivalent properties in the Cascading Style Sheets (CSS)
Most HTML authors find the strict XHTML DTD too restrictive because
many of the deprecated elements and attributes are still in widespread
use throughout the Web. More importantly, lots of content out there
on the Web uses the legacy elements and attributes, and the popular
browsers still support most of the deprecated elements. 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. [*]
[*] If the W3C has its way, HTML won't change beyond version 4.01. No more HTML; all new develop-
ments 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 wonder-
ful, but deprecated, features that make life as an HTML author easier.
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