... when we already have "normal" HTML?
- Access to the strengths of XML:
- Extensibility
- Interoperability
- Developers can extend XHTML
- Modules
- DTDs
- Schemas
- other XML-based languages
- Developers can subset XHTML
- XSLT Transformations
- Makes it easy to adapt pages for a wide variety of
user agents (such as browsers).
The value of XHTML 1.0 is that web documents can be
processed as XML. XHTML is the easy intersection between the
current web and XML.
The value of XHTML 1.1 is the ability to create XHTML
subsets, extensions and other variants. This
is the platform for extending HTML that people have wanted
for years. Murray Altheim, current W3C lead of the HTML
Working Group, writes:
... I've got a half dozen or so XHTML variants using the
XHTML 1.1 modular framework, versions that include SVG,
MathML, etc., a version with a DocBook superstructure.
I'm working with the HTML Writer's Guild on an XHTML-
based DTD for the Gutenberg Project ...
The value of XHTML 2.0 will be the change to XLink from
HTML's linking syntax. Still modular, with extended forms
capabilities and possibly more complex events.
No and yes. Current browser releases do not speak XHTML, they
speak Tag Soup masquerading as HTML (and HTML too, but that's by
accident). However, it is possible to write XHTML in a way that is
compatible with HTML, and will not break on existing HTML-based
browsers. These are described in Appendix C of the XHTML
specification.
As an example, <HR> is an empty element, and would
normally be written in XHTML as either <hr/> or
<hr></hr>.
Either will confuse many existing browsers; instead, you can write
the tag as <hr /> - note the space. This is
legitimate XHTML and is compatible with HTML browsers.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Hello World</title>
</head>
<body>
<p>Howdy doody.</p>
</body>
</html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>A Math Example</title>
</head>
<body>
<p>The following is MathML markup:</p>
<math xmlns="http://www.w3.org/1998/Math/MathML">
<apply> <log/>
<logbase>
<cn> 3 </cn>
</logbase>
<ci> x </ci>
</apply>
</math>
</body>
</html>
<book xmlns='urn:loc.gov:books'
xmlns:isbn='urn:ISBN:0-395-36341-6'
xml:lang="en">
<title>Cheaper by the Dozen</title>
<isbn:number>1568491379</isbn:number>
<notes>
<p xmlns='http://www.w3.org/1999/xhtml'>
This is also available <a href=
"http://www.w3.org/">online</a>.
</p>
</notes>
</book>
Database inclusions using XLink:
<html>
<a xlink:href="headstuff.xml" xlink:show="embed"
xlink:actuate="onLoad"/>
<body>
<a xlink:href="adbanner.xml" xlink:show="embed"
xlink:actuate="onLoad"/>
<a xlink:href="dataquery.xml" xlink:show="embed"
xlink:actuate="onLoad"/>
</body>
</html>
If you're already familiar with HTML 4.01, you'll find that XHTML
1.0 offers the same features and abilities you've always had.
If you observe a few simple syntactic rules and guidelines,
you'll be able to author XHTML 1.0 documents when you leave
here tonight.
Note that future versions of XHTML will expand beyond simply
"translating" the HTML specification into XML.
Means all elements must either have closing tags or be
written in a special form ("Empty", described shortly), and
that all the elements must nest.
Although overlapping is illegal in SGML, it was widely tolerated
in existing browsers.
INCORRECT: overlapping elements
<p>here is an emphasized
<em>paragraph.</p></em>
CORRECT: nested elements
<p>here is an emphasized
<em>paragraph</em>.</p>
In HTML, tags and attributes are not case sensitive. This
means that <BODY>,
<body>, <Body>, and even
<bOdY> refer to the same thing; you can
have your opening tag be <body> and your
closing tag </BODY>, and it all works
fine.
XML is case sensitive:
<Body> is considered a different tag from
<BODY>, <body>, or
<bOdY>.
Because of this, a standard for XHTML tags had to be set -
and the most likely choices were "all uppercase" (such as
<BODY>), or "all lowercase" (such as
<body>). In the end, lowercase won out,
although it really could have gone either way; a semi-
arbitrary choice had to be made.
So - element and attribute names must be in lower case. So
do attribute values listed in the DTD, as in <p
align="left"> (Transitional DTD
example).
In SGML-based HTML 4 certain elements were permitted to omit
the end tag; with the elements that followed implying closure.
This omission is not permitted in XML-based XHTML. All elements
other than those declared in the DTD as EMPTY must
have an end tag.
INCORRECT: unterminated elements
<p>here is a paragraph.<p>here is another
paragraph.
CORRECT: terminated elements
<p>here is a paragraph.</p><p>here is
another paragraph.</p>
All attribute values must be quoted, even those which appear
to be numeric.
INCORRECT: unquoted attribute values
<table border=0>
CORRECT: quoted attribute values
<table border="0">
XML does not support attribute minimization. Attribute-value
pairs must be written in full. Attribute names such as
compact and checked cannot occur in elements
without their value being specified.
INCORRECT: minimized attributes
<option selected>
CORRECT: unminimized attributes
<option selected="selected">
Empty elements must either have an end tag or the start tag
must end with />. For instance,
<br/> or <br></br>.
INCORRECT: unterminated empty tags
<br> <hr>
CORRECT: terminated empty tags
<br/> <hr />
In XHTML, the script and style elements are declared as having
#PCDATA content. As a result, < and
& will be treated as the start of markup, and
entities such as < and &
will be recognized as entity references by the XML processor to
< and & respectively. Wrapping
the content of the script or style element within a
CDATA marked section avoids the expansion of these
entities.
<script type="text/javascript"><![CDATA[
if (i < 5) && (j < 10)
alert("What sweet sorrow!");
]]></script>
An alternative is to use external script and style
documents!
HTML 4 defined the name attribute for the
elements a, applet, form,
frame, iframe, img, and
map. HTML 4 also introduced the id
attribute. Both of these attributes are designed to be used as
fragment identifiers.
In XML, fragment identifiers are of type ID, and
there can only be a single attribute of type ID per
element. In order to ensure that XHTML 1.0 documents are
well-structured XML documents, XHTML 1.0 documents MUST use
id when defining fragment identifiers, even on
elements that historically have also had a name
attribute.
Note that in XHTML 1.0, the name attribute of
these elements is formally deprecated, and will be removed in a
subsequent version of XHTML.
