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1<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "dtds/xhtml1-strict.dtd">
2<?xml-stylesheet href="W3C-PR.css" type="text/css"?>
3<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
4<head>
5<title>XHTML 1.0: The Extensible HyperText Markup
6Language</title>
7<link rel="stylesheet"
8href="W3C-PR.css" type="text/css" />
9<style type="text/css">
10span.term { font-style: italic; color: rgb(0, 0, 192) }
11code {
12 color: green;
13 font-family: monospace;
14 font-weight: bold;
15}
16
17code.greenmono {
18 color: green;
19 font-family: monospace;
20 font-weight: bold;
21}
22.good {
23 border: solid green;
24 border-width: 2px;
25 color: green;
26 font-weight: bold;
27 margin-right: 5%;
28 margin-left: 0;
29}
30.bad {
31 border: solid red;
32 border-width: 2px;
33 margin-left: 0;
34 margin-right: 5%;
35 color: rgb(192, 101, 101);
36}
37
38img {
39 color: white;
40 border: none;
41}
42
43div.navbar { text-align: center; }
44div.contents {
45 background-color: rgb(204,204,255);
46 padding: 0.5em;
47 border: none;
48 margin-right: 5%;
49}
50.tocline { list-style: none; }
51table.exceptions { background-color: rgb(255,255,153); }
52</style>
53</head>
54<body>
55<div class="navbar">
56 <a href="#toc">table of contents</a>
57 <hr />
58</div>
59<div class="head"><p><a href="http://www.w3.org/"><img class="head"
60src="w3c_home.gif" alt="W3C" /></a></p>
61
62<h1 class="head"><a name="title" id="title">XHTML</a><sup>&#8482;</sup> 1.0:
63The Extensible HyperText Markup Language</h1>
64
65<h2>A Reformulation of HTML 4.0 in XML 1.0</h2>
66
67<h3>W3C Proposed Recommendation 10 December 1999</h3>
68
69<dl>
70<dt>This version:</dt>
71
72<dd><a href=
73"http://www.w3.org/TR/1999/PR-xhtml1-19991210">
74http://www.w3.org/TR/1999/PR-xhtml1-19991210</a> <br />
75(<a href="xhtml1.ps">Postscript version</a>,
76<a href="xhtml1.pdf">PDF version</a>,
77<a href="xhtml1.zip">ZIP archive</a>, or
78<a href="xhtml1.tgz">Gzip'd TAR archive</a>)
79</dd>
80
81<dt>Latest version:</dt>
82
83<dd><a href="http://www.w3.org/TR/xhtml1">
84http://www.w3.org/TR/xhtml1</a></dd>
85
86<dt>Previous versions:</dt>
87
88<dd><a href=
89"http://www.w3.org/TR/1999/WD-xhtml1-19991124">
90http://www.w3.org/TR/1999/WD-xhtml1-19991124</a></dd>
91<dd><a href=
92"http://www.w3.org/TR/1999/PR-xhtml1-19990824">
93http://www.w3.org/TR/1999/PR-xhtml1-19990824</a></dd>
94
95<dt>Authors:</dt>
96
97<dd>See <a href="#acks">acknowledgements</a>.</dd>
98</dl>
99
100<p class="copyright"><a href=
101"http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">
102Copyright</a> &copy; 1999 <a href="http://www.w3.org/">W3C</a><sup>&reg;</sup>
103(<a href="http://www.lcs.mit.edu/">MIT</a>, <a href=
104"http://www.inria.fr/">INRIA</a>, <a href=
105"http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. <abbr
106title="World Wide Web Consortium">W3C</abbr> <a
107href=
108"http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">
109liability</a>, <a href=
110"http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">
111trademark</a>, <a href=
112"http://www.w3.org/Consortium/Legal/copyright-documents">document
113use</a> and <a href=
114"http://www.w3.org/Consortium/Legal/copyright-software">software
115licensing</a> rules apply.</p>
116<hr />
117</div>
118
119<h2 class="notoc">Abstract</h2>
120
121<p>This specification defines <abbr title="Extensible Hypertext Markup
122Language">XHTML</abbr> 1.0, a reformulation of HTML
1234.0 as an XML 1.0 application, and three <abbr title="Document Type
124Definition">DTDs</abbr> corresponding to
125the ones defined by HTML 4.0. The semantics of the elements and
126their attributes are defined in the W3C Recommendation for HTML
1274.0. These semantics provide the foundation for future
128extensibility of XHTML. Compatibility with existing HTML user
129agents is possible by following a small set of guidelines.</p>
130
131<h2>Status of this document</h2>
132
133<p><em>This section describes the status of this document at the time
134of its publication. Other documents may supersede this document. The
135latest status of this document series is maintained at the W3C.</em></p>
136
137<p>This specification is a Proposed Recommendation of the HTML Working Group. It is
138a revision of the Proposed Recommendation dated <a
139href= "http://www.w3.org/TR/1999/PR-xhtml1-19990824/">24 August
1401999</a> incorporating changes as a result of comments from the Proposed
141Recommendation review, and
142comments and further deliberations of the W3C HTML Working Group. A
143<a href="xhtml1-diff-19991210.html">diff-marked version</a> from the previous
144proposed recommendation is available for comparison purposes.</p>
145
146<p>On 10 December 1999, this document enters a
147<a href="http://www.w3.org/Consortium/Process/#RecsPR">
148Proposed Recommendation</a> review period. From that date until 8 January
1492000,
150W3C Advisory Committee representatives are encouraged
151to review this specification and return comments in their completed
152ballots to [email protected]. Please send any comments of a
153confidential nature in separate email to [email protected], which is
154visible to the Team only.</p>
155
156<p>No sooner than 14 days after the end of the review period, the
157Director will announce the document's disposition: it may become a W3C
158Recommendation (possibly with minor changes), it may revert to Working
159Draft status, or it may be dropped as a W3C work item.</p>
160
161<p>Publication as a Proposed Recommendation does not imply endorsement
162by the W3C membership. This is still a draft document and may be
163updated, replaced or obsoleted by other documents at any time. It is
164inappropriate to cite W3C Proposed Recommendation as other than "work
165in progress."</p>
166
167<p>This document has been produced as part of the <a href=
168"http://www.w3.org/MarkUp/">W3C HTML Activity</a>. The goals of
169the <a href="http://www.w3.org/MarkUp/Group/">HTML Working
170Group</a> <i>(<a href="http://cgi.w3.org/MemberAccess/">members
171only</a>)</i> are discussed in the <a href=
172"http://www.w3.org/MarkUp/Group/HTMLcharter">HTML Working Group
173charter</a> <i>(<a href="http://cgi.w3.org/MemberAccess/">members
174only</a>)</i>.</p>
175
176<p>A list of current W3C Recommendations and other technical documents
177can be found at <a
178href="http://www.w3.org/TR">http://www.w3.org/TR</a>.</p>
179
180<p>Public discussion on <abbr title="HyperText Markup
181Language">HTML</abbr> features takes place on the mailing list <a
182href="mailto:[email protected]"> [email protected]</a> (<a href=
183"http://lists.w3.org/Archives/Public/www-html/">archive</a>). The W3C
184staff contact for work on HTML is <a href= "mailto:[email protected]">Dave
185Raggett</a>.</p>
186
187<p>Please report errors in this document to <a
188href="mailto:[email protected]">[email protected]</a>.</p>
189
190<p>The list of known errors in this specification is available at <a
191href="http://www.w3.org/1999/12/PR-xhtml1-19991210-errata">http://www.w3.org/1999/12/PR-xhtml1-19991210-errata</a>.</p>
192
193<h2 class="notoc"><a id="toc" name="toc">Contents</a></h2>
194
195<div class="contents">
196<ul class="toc">
197<li class="tocline">1. <a href="#xhtml">What is XHTML?</a>
198
199<ul class="toc">
200<li class="tocline">1.1 <a href="#html4">What is HTML 4.0?</a></li>
201
202<li class="tocline">1.2 <a href="#xml">What is XML?</a></li>
203
204<li class="tocline">1.3 <a href="#why">Why the need for XHTML?</a></li>
205</ul>
206</li>
207
208<li class="tocline">2. <a href="#defs">Definitions</a>
209
210<ul class="toc">
211<li class="tocline">2.1 <a href="#terms">Terminology</a></li>
212
213<li class="tocline">2.2 <a href="#general">General Terms</a></li>
214</ul>
215</li>
216
217<li class="tocline">3. <a href="#normative">Normative Definition of XHTML 1.0</a>
218
219
220<ul class="toc">
221<li class="tocline">3.1 <a href="#docconf">Document Conformance</a></li>
222
223<li class="tocline">3.2 <a href="#uaconf">User Agent Conformance</a></li>
224</ul>
225</li>
226
227<li class="tocline">4. <a href="#diffs">Differences with HTML 4.0</a>
228
229</li>
230
231<li class="tocline">5. <a href="#issues">Compatibility Issues</a>
232
233<ul class="toc">
234<li class="tocline">5.1 <a href="#media">Internet Media Types</a></li>
235</ul>
236</li>
237
238<li class="tocline">6. <a href="#future">Future Directions</a>
239
240<ul class="toc">
241<li class="tocline">6.1 <a href="#mods">Modularizing HTML</a></li>
242
243<li class="tocline">6.2 <a href="#extensions">Subsets and Extensibility</a></li>
244
245<li class="tocline">6.3 <a href="#profiles">Document Profiles</a></li>
246</ul>
247</li>
248
249<li class="tocline"><a href="#dtds">Appendix A. DTDs</a></li>
250
251<li class="tocline"><a href="#prohibitions">Appendix B. Element
252Prohibitions</a></li>
253
254<li class="tocline"><a href="#guidelines">Appendix C. HTML Compatibility Guidelines</a></li>
255
256<li class="tocline"><a href="#acks">Appendix D. Acknowledgements</a></li>
257
258<li class="tocline"><a href="#refs">Appendix E. References</a></li>
259</ul>
260</div>
261
262<!--OddPage-->
263<h1><a name="xhtml" id="xhtml">1. What is XHTML?</a></h1>
264
265<p>XHTML is a family of current and future document types and modules that
266reproduce, subset, and extend HTML 4.0 <a href="#ref-html4">[HTML]</a>. XHTML family document types are <abbr title="Extensible Markup Language">XML</abbr> based,
267and ultimately are designed to work in conjunction with XML-based user agents.
268The details of this family and its evolution are
269discussed in more detail in the section on <a href="#future">Future
270Directions</a>. </p>
271
272<p>XHTML 1.0 (this specification) is the first document type in the XHTML
273family. It is a reformulation of the three HTML 4.0 document types as
274applications of XML 1.0 <a href="#ref-xml"> [XML]</a>. It is intended
275to be used as a language for content that is both XML-conforming and, if some
276simple <a href="#guidelines">guidelines</a> are followed,
277operates in HTML 4.0 conforming user agents. Developers who migrate
278their content to XHTML 1.0 will realize the following benefits:</p>
279
280<ul>
281<li>XHTML documents are XML conforming. As such, they are readily viewed,
282edited, and validated with standard XML tools.</li>
283<li>XHTML documents can be written to
284to operate as well or better than they did before in existing
285HTML 4.0-conforming user agents as well as in new, XHTML 1.0 conforming user
286agents.</li>
287<li>XHTML documents can utilize applications (e.g. scripts and applets) that rely
288upon either the HTML Document Object Model or the XML Document Object Model <a
289href="#ref-dom">[DOM]</a>.</li>
290<li>As the XHTML family evolves, documents conforming to XHTML 1.0 will be more
291likely to interoperate within and among various XHTML environments.</li>
292</ul>
293
294<p>The XHTML family is the next step in the evolution of the Internet. By
295migrating to XHTML today, content developers can enter the XML world with all
296of its attendant benefits, while still remaining confident in their
297content's backward and future compatibility.</p>
298
299<h2><a name="html4" id="html4">1.1 What is HTML 4.0?</a></h2>
300
301<p>HTML 4.0 <a href="#ref-html4">[HTML]</a> is an <abbr title="Standard
302Generalized Markup Language">SGML</abbr> (Standard
303Generalized Markup Language) application conforming to
304International Standard <abbr title="Organization for International
305Standardization">ISO</abbr> 8879, and is widely regarded as the
306standard publishing language of the World Wide Web.</p>
307
308<p>SGML is a language for describing markup languages,
309particularly those used in electronic document exchange, document
310management, and document publishing. HTML is an example of a
311language defined in SGML.</p>
312
313<p>SGML has been around since the middle 1980's and has remained
314quite stable. Much of this stability stems from the fact that the
315language is both feature-rich and flexible. This flexibility,
316however, comes at a price, and that price is a level of
317complexity that has inhibited its adoption in a diversity of
318environments, including the World Wide Web.</p>
319
320<p>HTML, as originally conceived, was to be a language for the
321exchange of scientific and other technical documents, suitable
322for use by non-document specialists. HTML addressed the problem
323of SGML complexity by specifying a small set of structural and
324semantic tags suitable for authoring relatively simple documents.
325In addition to simplifying the document structure, HTML added
326support for hypertext. Multimedia capabilities were added
327later.</p>
328
329<p>In a remarkably short space of time, HTML became wildly
330popular and rapidly outgrew its original purpose. Since HTML's
331inception, there has been rapid invention of new elements for use
332within HTML (as a standard) and for adapting HTML to vertical,
333highly specialized, markets. This plethora of new elements has
334led to compatibility problems for documents across different
335platforms.</p>
336
337<p>As the heterogeneity of both software and platforms rapidly
338proliferate, it is clear that the suitability of 'classic' HTML
3394.0 for use on these platforms is somewhat limited.</p>
340
341<h2><a name="xml" id="xml">1.2 What is XML?</a></h2>
342
343<p>XML<sup>&#8482;</sup> is the shorthand for Extensible Markup
344Language, and is an acronym of Extensible Markup Language <a
345href="#ref-xml">[XML]</a>.</p>
346
347<p>XML was conceived as a means of regaining the power and
348flexibility of SGML without most of its complexity. Although a
349restricted form of SGML, XML nonetheless preserves most of SGML's
350power and richness, and yet still retains all of SGML's commonly
351used features.</p>
352
353<p>While retaining these beneficial features, XML removes many of
354the more complex features of SGML that make the authoring and
355design of suitable software both difficult and costly.</p>
356
357<h2><a name="why" id="why">1.3 Why the need for XHTML?</a></h2>
358
359<p>The benefits of migrating to XHTML 1.0 are described above. Some of the
360benefits of migrating to XHTML in general are:</p>
361
362<ul>
363<li>Document developers and user agent designers are constantly
364discovering new ways to express their ideas through new markup. In XML, it is
365relatively easy to introduce new elements or additional element
366attributes. The XHTML family is designed to accommodate these extensions
367through XHTML modules and techniques for developing new XHTML-conforming
368modules (described in the forthcoming XHTML Modularization specification).
369These modules will permit the combination of existing and
370new feature sets when developing content and when designing new user
371agents.</li>
372
373<li>Alternate ways of accessing the Internet are constantly being
374introduced. Some estimates indicate that by the year 2002, 75% of
375Internet document viewing will be carried out on these alternate
376platforms. The XHTML family is designed with general user agent
377interoperability in mind. Through a new user agent and document profiling
378mechanism, servers, proxies, and user agents will be able to perform
379best effort content transformation. Ultimately, it will be possible to
380develop XHTML-conforming content that is usable by any XHTML-conforming
381user agent.</li>
382
383</ul>
384<!--OddPage-->
385<h1><a name="defs" id="defs">2. Definitions</a></h1>
386
387<h2><a name="terms" id="terms">2.1 Terminology</a></h2>
388
389<p>The following terms are used in this specification. These
390terms extend the definitions in <a href="#ref-rfc2119">
391[RFC2119]</a> in ways based upon similar definitions in ISO/<abbr
392title="International Electro-technical Commission">IEC</abbr>
3939945-1:1990 <a href="#ref-posix">[POSIX.1]</a>:</p>
394
395<dl>
396<dt>Implementation-defined</dt>
397
398<dd>A value or behavior is implementation-defined when it is left
399to the implementation to define [and document] the corresponding
400requirements for correct document construction.</dd>
401
402<dt>May</dt>
403
404<dd>With respect to implementations, the word "may" is to be
405interpreted as an optional feature that is not required in this
406specification but can be provided. With respect to <a href=
407"#docconf">Document Conformance</a>, the word "may" means that
408the optional feature must not be used. The term "optional" has
409the same definition as "may".</dd>
410
411<dt>Must</dt>
412
413<dd>In this specification, the word "must" is to be interpreted
414as a mandatory requirement on the implementation or on Strictly
415Conforming XHTML Documents, depending upon the context. The term
416"shall" has the same definition as "must".</dd>
417
418<dt>Reserved</dt>
419
420<dd>A value or behavior is unspecified, but it is not allowed to
421be used by Conforming Documents nor to be supported by a
422Conforming User Agents.</dd>
423
424<dt>Should</dt>
425
426<dd>With respect to implementations, the word "should" is to be
427interpreted as an implementation recommendation, but not a
428requirement. With respect to documents, the word "should" is to
429be interpreted as recommended programming practice for documents
430and a requirement for Strictly Conforming XHTML Documents.</dd>
431
432<dt>Supported</dt>
433
434<dd>Certain facilities in this specification are optional. If a
435facility is supported, it behaves as specified by this
436specification.</dd>
437
438<dt>Unspecified</dt>
439
440<dd>When a value or behavior is unspecified, the specification
441defines no portability requirements for a facility on an
442implementation even when faced with a document that uses the
443facility. A document that requires specific behavior in such an
444instance, rather than tolerating any behavior when using that
445facility, is not a Strictly Conforming XHTML Document.</dd>
446</dl>
447
448<h2><a name="general" id="general">2.2 General Terms</a></h2>
449
450<dl>
451<dt>Attribute</dt>
452
453<dd>An attribute is a parameter to an element declared in the
454DTD. An attribute's type and value range, including a possible
455default value, are defined in the DTD.</dd>
456
457<dt>DTD</dt>
458
459<dd>A DTD, or document type definition, is a collection of XML
460declarations that, as a collection, defines the legal structure,
461<span class="term">elements</span>, and <span class="term">
462attributes</span> that are available for use in a document that
463complies to the DTD.</dd>
464
465<dt>Document</dt>
466
467<dd>A document is a stream of data that, after being combined
468with any other streams it references, is structured such that it
469holds information contained within <span class="term">
470elements</span> that are organized as defined in the associated
471<span class="term">DTD</span>. See <a href="#docconf">Document
472Conformance</a> for more information.</dd>
473
474<dt>Element</dt>
475
476<dd>An element is a document structuring unit declared in the
477<span class="term">DTD</span>. The element's content model is
478defined in the <span class="term">DTD</span>, and additional
479semantics may be defined in the prose description of the
480element.</dd>
481
482<dt><a name="facilities" id="facilities">Facilities</a></dt>
483
484<dd>Functionality includes <span class="term">elements</span>,
485<span class="term">attributes</span>, and the semantics
486associated with those <span class="term">elements</span> and
487<span class="term">attributes</span>. An implementation
488supporting that functionality is said to provide the necessary
489facilities.</dd>
490
491<dt>Implementation</dt>
492
493<dd>An implementation is a system that provides collection of
494<span class="term">facilities</span> and services that supports
495this specification. See <a href="#uaconf">User Agent
496Conformance</a> for more information.</dd>
497
498<dt>Parsing</dt>
499
500<dd>Parsing is the act whereby a <span class="term">
501document</span> is scanned, and the information contained within
502the <span class="term">document</span> is filtered into the
503context of the <span class="term">elements</span> in which the
504information is structured.</dd>
505
506<dt>Rendering</dt>
507
508<dd>Rendering is the act whereby the information in a <span
509class="term">document</span> is presented. This presentation is
510done in the form most appropriate to the environment (e.g.
511aurally, visually, in print).</dd>
512
513<dt>User Agent</dt>
514
515<dd>A user agent is an <span class="term">implementation</span>
516that retrieves and processes XHTML documents. See <a href=
517"#uaconf">User Agent Conformance</a> for more information.</dd>
518
519<dt>Validation</dt>
520
521<dd>Validation is a process whereby <span class="term">
522documents</span> are verified against the associated <span class=
523"term">DTD</span>, ensuring that the structure, use of <span
524class="term">elements</span>, and use of <span class="term">
525attributes</span> are consistent with the definitions in the
526<span class="term">DTD</span>.</dd>
527
528<dt><a name="wellformed" id="wellformed">Well-formed</a></dt>
529
530<dd>A <span class="term">document</span> is well-formed when it
531is structured according to the rules defined in <a href=
532"http://www.w3.org/TR/REC-xml#sec-well-formed">Section 2.1</a> of
533the XML 1.0 Recommendation <a href="#ref-xml">[XML]</a>.
534Basically, this definition states that elements, delimited by
535their start and end tags, are nested properly within one
536another.</dd>
537</dl>
538
539<!--OddPage-->
540<h1><a name="normative" id="normative">3. Normative Definition of
541XHTML 1.0</a></h1>
542
543<h2><a name="docconf" id="docconf">3.1 Document
544Conformance</a></h2>
545
546<p>This version of XHTML provides a definition of strictly
547conforming XHTML documents, which are restricted to tags and
548attributes from the XHTML namespace. See <a href=
549"#well-formed">Section 3.1.2</a> for information on using XHTML
550with other namespaces, for instance, to include metadata
551expressed in <abbr title="Resource Description Format">RDF</abbr> within XHTML documents.</p>
552
553<h3><a name="strict" id="strict">3.1.1 Strictly Conforming
554Documents</a></h3>
555
556<p>A Strictly Conforming XHTML Document is a document that
557requires only the facilities described as mandatory in this
558specification. Such a document must meet all of the following
559criteria:</p>
560
561<ol>
562<li>
563<p>It must validate against one of the three DTDs found in <a
564href="#dtds">Appendix&#160;A</a>.</p>
565</li>
566
567<li>
568<p>The root element of the document must be <code>
569&lt;html&gt;</code>.</p>
570</li>
571
572<li>
573<p>The root element of the document must designate the XHTML
574namespace using the <code>xmlns</code> attribute <a href=
575"#ref-xmlns">[XMLNAMES]</a>. The namespace for XHTML is
576defined to be
577<code>http://www.w3.org/1999/xhtml</code>.</p>
578</li>
579
580<li>
581<p>There must be a DOCTYPE declaration in the document prior to
582the root element. The public identifier included in
583the DOCTYPE declaration must reference one of the three DTDs
584found in <a href="#dtds">Appendix&#160;A</a> using the respective
585Formal Public Identifier. The system identifier may be changed to reflect
586local system conventions.</p>
587
588<pre>
589&lt;!DOCTYPE html
590 PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
591 "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-strict.dtd&gt;
592
593&lt;!DOCTYPE html
594 PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
595 "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-transitional.dtd&gt;
596
597&lt;!DOCTYPE html
598 PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
599 "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-frameset.dtd&gt;
600</pre>
601</li>
602</ol>
603
604<p>Here is an example of a minimal XHTML document.</p>
605
606<div class="good">
607<pre>
608&lt;?xml version="1.0" encoding="UTF-8"?&gt;
609&lt;!DOCTYPE html
610 PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
611 "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-strict.dtd"&gt;
612&lt;html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"&gt;
613 &lt;head&gt;
614 &lt;title&gt;Virtual Library&lt;/title&gt;
615 &lt;/head&gt;
616 &lt;body&gt;
617 &lt;p&gt;Moved to &lt;a href="http://vlib.org/"&gt;vlib.org&lt;/a&gt;.&lt;/p&gt;
618 &lt;/body&gt;
619&lt;/html&gt;</pre>
620</div>
621
622<p>Note that in this example, the XML declaration is included. An XML
623declaration like the one above is
624not required in all XML documents. XHTML document authors are strongly encouraged to use XML declarations in all their documents. Such a declaration is required
625when the character encoding of the document is other than the default UTF-8 or
626UTF-16.</p>
627
628<h3><a name="well-formed" id="well-formed">3.1.2 Using XHTML with
629other namespaces</a></h3>
630
631<p>The XHTML namespace may be used with other XML namespaces
632as per <a href="#ref-xmlns">[XMLNAMES]</a>, although such
633documents are not strictly conforming XHTML 1.0 documents as
634defined above. Future work by W3C will address ways to specify
635conformance for documents involving multiple namespaces.</p>
636
637<p>The following example shows the way in which XHTML 1.0 could
638be used in conjunction with the MathML Recommendation:</p>
639
640<div class="good">
641<pre>
642&lt;html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"&gt;
643 &lt;head&gt;
644 &lt;title&gt;A Math Example&lt;/title&gt;
645 &lt;/head&gt;
646 &lt;body&gt;
647 &lt;p&gt;The following is MathML markup:&lt;/p&gt;
648 &lt;math xmlns="http://www.w3.org/1998/Math/MathML"&gt;
649 &lt;apply&gt; &lt;log/&gt;
650 &lt;logbase&gt;
651 &lt;cn&gt; 3 &lt;/cn&gt;
652 &lt;/logbase&gt;
653 &lt;ci&gt; x &lt;/ci&gt;
654 &lt;/apply&gt;
655 &lt;/math&gt;
656 &lt;/body&gt;
657&lt;/html&gt;
658</pre>
659</div>
660
661<p>The following example shows the way in which XHTML 1.0 markup
662could be incorporated into another XML namespace:</p>
663
664<div class="good">
665<pre>
666&lt;?xml version="1.0" encoding="UTF-8"?&gt;
667&lt;!-- initially, the default namespace is "books" --&gt;
668&lt;book xmlns='urn:loc.gov:books'
669 xmlns:isbn='urn:ISBN:0-395-36341-6' xml:lang="en" lang="en"&gt;
670 &lt;title&gt;Cheaper by the Dozen&lt;/title&gt;
671 &lt;isbn:number&gt;1568491379&lt;/isbn:number&gt;
672 &lt;notes&gt;
673 &lt;!-- make HTML the default namespace for a hypertext commentary --&gt;
674 &lt;p xmlns='http://www.w3.org/1999/xhtml'&gt;
675 This is also available &lt;a href="http://www.w3.org/"&gt;online&lt;/a&gt;.
676 &lt;/p&gt;
677 &lt;/notes&gt;
678&lt;/book&gt;
679</pre>
680</div>
681
682<h2><a name="uaconf" id="uaconf">3.2 User Agent
683Conformance</a></h2>
684
685<p>A conforming user agent must meet all of the following
686criteria:</p>
687
688<ol>
689<li>In order to be consistent with the XML 1.0 Recommendation <a
690href="#ref-xml">[XML]</a>, the user agent must parse and evaluate
691an XHTML document for well-formedness. If the user agent claims
692to be a validating user agent, it must also validate documents
693against their referenced DTDs according to <a href="#ref-xml">
694[XML]</a>.</li>
695
696<li>When the user agent claims to support <a href="#facilities">
697facilities</a> defined within this specification or required by
698this specification through normative reference, it must do so in
699ways consistent with the facilities' definition.</li>
700
701<li>When a user agent processes an XHTML document as generic XML,
702it shall only recognize attributes of type
703<code>ID</code> (e.g. the <code>id</code> attribute on most XHTML elements)
704as fragment identifiers.</li>
705
706<li>If a user agent encounters an element it does not recognize,
707it must render the element's content.</li>
708
709<li>If a user agent encounters an attribute it does not
710recognize, it must ignore the entire attribute specification
711(i.e., the attribute and its value).</li>
712
713<li>If a user agent encounters an attribute value it doesn't
714recognize, it must use the default attribute value.</li>
715
716<li>If it encounters an entity reference (other than one
717of the predefined entities) for which the User Agent has
718processed no declaration (which could happen if the declaration
719is in the external subset which the User Agent hasn't read), the entity
720reference should be rendered as the characters (starting
721with the ampersand and ending with the semi-colon) that
722make up the entity reference.</li>
723
724<li>When rendering content, User Agents that encounter
725characters or character entity references that are recognized but not renderable should display the document in such a way that it is obvious to the user that normal rendering has not taken place.</li>
726
727<li>
728The following characters are defined in [XML] as whitespace characters:
729
730<ul>
731<li>Space (&amp;#x0020;)</li>
732<li>Tab (&amp;#x0009;)</li>
733<li>Carriage return (&amp;#x000D;)</li>
734<li>Line feed (&amp;#x000A;)</li>
735</ul>
736
737<p>
738The XML processor normalizes different system's line end codes into one
739single line-feed character, that is passed up to the application. The XHTML
740user agent in addition, must treat the following characters as whitespace:
741</p>
742
743<ul>
744<li>Form feed (&amp;#x000C;)</li>
745<li>Zero-width space (&amp;#x200B;)</li>
746</ul>
747
748<p>
749In elements where the 'xml:space' attribute is set to 'preserve', the user
750agent must leave all whitespace characters intact (with the exception of
751leading and trailing whitespace characters, which should be removed).
752Otherwise, whitespace
753is handled according to the following rules:
754</p>
755
756<ul>
757<li>
758All whitespace surrounding block elements should be removed.
759</li>
760<li>
761Comments are removed entirely and do not affect whitespace handling. One
762whitespace character on either side of a comment is treated as two white
763space characters.
764</li>
765<li>
766Leading and trailing whitespace inside a block element must be removed.
767</li>
768<li>Line feed characters within a block element must be converted into a
769space (except when the 'xml:space' attribute is set to 'preserve').
770</li>
771<li>
772A sequence of white space characters must be reduced to a single space
773character (except when the 'xml:space' attribute is set to 'preserve').
774</li>
775<li>
776With regard to rendition,
777the User Agent should render the content in a
778manner appropriate to the language in which the content is written.
779In languages whose primary script is Latinate, the ASCII space
780character is typically used to encode both grammatical word boundaries and
781typographic whitespace; in languages whose script is related to Nagari
782(e.g., Sanskrit, Thai, etc.), grammatical boundaries may be encoded using
783the ZW 'space' character, but will not typically be represented by
784typographic whitespace in rendered output; languages using Arabiform scripts
785may encode typographic whitespace using a space character, but may also use
786the ZW space character to delimit 'internal' grammatical boundaries (what
787look like words in Arabic to an English eye frequently encode several words,
788e.g. 'kitAbuhum' = 'kitAbu-hum' = 'book them' == their book); and languages
789in the Chinese script tradition typically neither encode such delimiters nor
790use typographic whitespace in this way.
791</li>
792</ul>
793
794<p>Whitespace in attribute values is processed according to <a
795href="#ref-xml">[XML]</a>.</p>
796</li>
797</ol>
798
799<!--OddPage-->
800<h1><a name="diffs" id="diffs">4. Differences with HTML
8014.0</a></h1>
802
803<p>Due to the fact that XHTML is an XML application, certain
804practices that were perfectly legal in SGML-based HTML 4.0 <a
805href="#ref-html4">[HTML]</a> must be changed.</p>
806
807<h2><a name="h-4.1" id="h-4.1">4.1 Documents must be
808well-formed</a></h2>
809
810<p><a href="#wellformed">Well-formedness</a> is a new concept
811introduced by <a href="#ref-xml">[XML]</a>. Essentially this
812means that all elements must either have closing tags or be
813written in a special form (as described below), and that all the
814elements must nest.</p>
815
816<p>Although overlapping is illegal in SGML, it was widely
817tolerated in existing browsers.</p>
818
819<div class="good">
820<p><strong><em>CORRECT: nested elements.</em></strong></p>
821
822<p>&lt;p&gt;here is an emphasized
823&lt;em&gt;paragraph&lt;/em&gt;.&lt;/p&gt;</p>
824</div>
825
826<div class="bad">
827<p><strong><em>INCORRECT: overlapping elements</em></strong></p>
828
829<p>&lt;p&gt;here is an emphasized
830&lt;em&gt;paragraph.&lt;/p&gt;&lt;/em&gt;</p>
831</div>
832
833<h2><a name="h-4.2" id="h-4.2">4.2 Element and attribute
834names must be in lower case</a></h2>
835
836<p>XHTML documents must use lower case for all HTML element and
837attribute names. This difference is necessary because XML is
838case-sensitive e.g. &lt;li&gt; and &lt;LI&gt; are different
839tags.</p>
840
841<h2><a name="h-4.3" id="h-4.3">4.3 For non-empty elements,
842end tags are required</a></h2>
843
844<p>In SGML-based HTML 4.0 certain elements were permitted to omit
845the end tag; with the elements that followed implying closure.
846This omission is not permitted in XML-based XHTML. All elements
847other than those declared in the DTD as <code>EMPTY</code> must
848have an end tag.</p>
849
850<div class="good">
851<p><strong><em>CORRECT: terminated elements</em></strong></p>
852
853<p>&lt;p&gt;here is a paragraph.&lt;/p&gt;&lt;p&gt;here is
854another paragraph.&lt;/p&gt;</p>
855</div>
856
857<div class="bad">
858<p><strong><em>INCORRECT: unterminated elements</em></strong></p>
859
860<p>&lt;p&gt;here is a paragraph.&lt;p&gt;here is another
861paragraph.</p>
862</div>
863
864<h2><a name="h-4.4" id="h-4.4">4.4 Attribute values must
865always be quoted</a></h2>
866
867<p>All attribute values must be quoted, even those which appear
868to be numeric.</p>
869
870<div class="good">
871<p><strong><em>CORRECT: quoted attribute values</em></strong></p>
872
873<p>&lt;table rows="3"&gt;</p>
874</div>
875
876<div class="bad">
877<p><strong><em>INCORRECT: unquoted attribute values</em></strong></p>
878
879<p>&lt;table rows=3&gt;</p>
880</div>
881
882<h2><a name="h-4.5" id="h-4.5">4.5 Attribute
883Minimization</a></h2>
884
885<p>XML does not support attribute minimization. Attribute-value
886pairs must be written in full. Attribute names such as <code>
887compact</code> and <code>checked</code> cannot occur in elements
888without their value being specified.</p>
889
890<div class="good">
891<p><strong><em>CORRECT: unminimized attributes</em></strong></p>
892
893<p>&lt;dl compact="compact"&gt;</p>
894</div>
895
896<div class="bad">
897<p><strong><em>INCORRECT: minimized attributes</em></strong></p>
898
899<p>&lt;dl compact&gt;</p>
900</div>
901
902<h2><a name="h-4.6" id="h-4.6">4.6 Empty Elements</a></h2>
903
904<p>Empty elements must either have an end tag or the start tag must end with <code>/&gt;</code>. For instance,
905<code>&lt;br/&gt;</code> or <code>&lt;hr&gt;&lt;/hr&gt;</code>. See <a
906href="#guidelines">HTML Compatibility Guidelines</a> for information on ways to
907ensure this is backward compatible with HTML 4.0 user agents.</p>
908
909<div class="good">
910<p><strong><em>CORRECT: terminated empty tags</em></strong></p>
911
912<p>&lt;br/&gt;&lt;hr/&gt;</p>
913</div>
914
915<div class="bad">
916<p><strong><em>INCORRECT: unterminated empty tags</em></strong></p>
917
918<p>&lt;br&gt;&lt;hr&gt;</p>
919</div>
920
921<h2><a name="h-4.7" id="h-4.7">4.7 Whitespace handling in
922attribute values</a></h2>
923
924<p>In attribute values, user agents will strip leading and
925trailing whitespace from attribute values and map sequences
926of one or more whitespace characters (including line breaks) to
927a single inter-word space (an ASCII space character for western
928scripts). See <a href="http://www.w3.org/TR/REC-xml#AVNormalize">
929Section 3.3.3</a> of <a href="#ref-xml">[XML]</a>.</p>
930
931<h2><a name="h-4.8" id="h-4.8">4.8 Script and Style
932elements</a></h2>
933
934<p>In XHTML, the script and style elements are declared as having
935<code>#PCDATA</code> content. As a result, <code>&lt;</code> and
936<code>&amp;</code> will be treated as the start of markup, and
937entities such as <code>&amp;lt;</code> and <code>&amp;amp;</code>
938will be recognized as entity references by the XML processor to
939<code>&lt;</code> and <code>&amp;</code> respectively. Wrapping
940the content of the script or style element within a <code>
941CDATA</code> marked section avoids the expansion of these
942entities.</p>
943
944<div class="good">
945<pre>
946&lt;script&gt;
947 &lt;![CDATA[
948 ... unescaped script content ...
949 ]]&gt;
950 &lt;/script&gt;
951</pre>
952</div>
953
954<p><code>CDATA</code> sections are recognized by the XML
955processor and appear as nodes in the Document Object Model, see
956<a href=
957"http://www.w3.org/TR/REC-DOM-Level-1/level-one-core.html#ID-E067D597">
958Section 1.3</a> of the DOM Level 1 Recommendation <a href=
959"#ref-dom">[DOM]</a>.</p>
960
961<p>An alternative is to use external script and style
962documents.</p>
963
964<h2><a name="h-4.9" id="h-4.9">4.9 SGML exclusions</a></h2>
965
966<p>SGML gives the writer of a DTD the ability to exclude specific
967elements from being contained within an element. Such
968prohibitions (called "exclusions") are not possible in XML.</p>
969
970<p>For example, the HTML 4.0 Strict DTD forbids the nesting of an
971'<code>a</code>' element within another '<code>a</code>' element
972to any descendant depth. It is not possible to spell out such
973prohibitions in XML. Even though these prohibitions cannot be
974defined in the DTD, certain elements should not be nested. A
975summary of such elements and the elements that should not be
976nested in them is found in the normative <a href="#prohibitions">
977Appendix&#160;B</a>.</p>
978
979<h2><a name="h-4.10" id="h-4.10">4.10 The elements with 'id' and 'name'
980attributes</a></h2>
981
982<p>HTML 4.0 defined the <code>name</code> attribute for the elements
983<code>a</code>,
984<code>applet</code>, <code>frame</code>,
985<code>iframe</code>, <code>img</code>, and <code>map</code>.
986HTML 4.0 also introduced
987the <code>id</code> attribute. Both of these attributes are designed to be
988used as fragment identifiers.</p>
989<p>In XML, fragment identifiers are of type <code>ID</code>, and
990there can only be a single attribute of type <code>ID</code> per element.
991Therefore, in XHTML 1.0 the <code>id</code>
992attribute is defined to be of type <code>ID</code>. In order to
993ensure that XHTML 1.0 documents are well-structured XML documents, XHTML 1.0
994documents MUST use the <code>id</code> attribute when defining fragment
995identifiers, even on elements that historically have also had a
996<code>name</code> attribute.
997See the <a href="#guidelines">HTML Compatibility
998Guidelines</a> for information on ensuring such anchors are backwards
999compatible when serving XHTML documents as media type <code>text/html</code>.
1000</p>
1001<p>Note that in XHTML 1.0, the <code>name</code> attribute of these
1002elements is formally deprecated, and will be removed in a
1003subsequent version of XHTML.</p>
1004
1005<!--OddPage-->
1006<h1><a name="issues" id="issues">5. Compatibility Issues</a></h1>
1007
1008<p>Although there is no requirement for XHTML 1.0 documents to be
1009compatible with existing user agents, in practice this is easy to
1010accomplish. Guidelines for creating compatible documents can be
1011found in <a href="#guidelines">Appendix&#160;C</a>.</p>
1012
1013<h2><a name="media" id="media">5.1 Internet Media Type</a></h2>
1014<p>As of the publication of this recommendation, the general
1015recommended MIME labeling for XML-based applications
1016has yet to be resolved.</p>
1017
1018<p>However, XHTML Documents which follow the guidelines set forth
1019in <a href="#guidelines">Appendix C</a>, "HTML Compatibility Guidelines" may be
1020labeled with the Internet Media Type "text/html", as they
1021are compatible with most HTML browsers. This document
1022makes no recommendation about MIME labeling of other
1023XHTML documents.</p>
1024
1025<!--OddPage-->
1026<h1><a name="future" id="future">6. Future Directions</a></h1>
1027
1028<p>XHTML 1.0 provides the basis for a family of document types
1029that will extend and subset XHTML, in order to support a wide
1030range of new devices and applications, by defining modules and
1031specifying a mechanism for combining these modules. This
1032mechanism will enable the extension and sub-setting of XHTML 1.0
1033in a uniform way through the definition of new modules.</p>
1034
1035<h2><a name="mods" id="mods">6.1 Modularizing HTML</a></h2>
1036
1037<p>As the use of XHTML moves from the traditional desktop user
1038agents to other platforms, it is clear that not all of the XHTML
1039elements will be required on all platforms. For example a hand
1040held device or a cell-phone may only support a subset of XHTML
1041elements.</p>
1042
1043<p>The process of modularization breaks XHTML up into a series of
1044smaller element sets. These elements can then be recombined to
1045meet the needs of different communities.</p>
1046
1047<p>These modules will be defined in a later W3C document.</p>
1048
1049<h2><a name="extensions" id="extensions">6.2 Subsets and
1050Extensibility</a></h2>
1051
1052<p>Modularization brings with it several advantages:</p>
1053
1054<ul>
1055<li>
1056<p>It provides a formal mechanism for sub-setting XHTML.</p>
1057</li>
1058
1059<li>
1060<p>It provides a formal mechanism for extending XHTML.</p>
1061</li>
1062
1063<li>
1064<p>It simplifies the transformation between document types.</p>
1065</li>
1066
1067<li>
1068<p>It promotes the reuse of modules in new document types.</p>
1069</li>
1070</ul>
1071
1072<h2><a name="profiles" id="profiles">6.3 Document
1073Profiles</a></h2>
1074
1075<p>A document profile specifies the syntax and semantics of a set
1076of documents. Conformance to a document profile provides a basis
1077for interoperability guarantees. The document profile specifies
1078the facilities required to process documents of that type, e.g.
1079which image formats can be used, levels of scripting, style sheet
1080support, and so on.</p>
1081
1082<p>For product designers this enables various groups to define
1083their own standard profile.</p>
1084
1085<p>For authors this will obviate the need to write several
1086different versions of documents for different clients.</p>
1087
1088<p>For special groups such as chemists, medical doctors, or
1089mathematicians this allows a special profile to be built using
1090standard HTML elements plus a group of elements geared to the
1091specialist's needs.</p>
1092
1093<!--OddPage-->
1094<h1><a name="appendices" id="appendices"></a>
1095<a name="dtds" id="dtds">Appendix A. DTDs</a></h1>
1096
1097<p><b>This appendix is normative.</b></p>
1098
1099<p>These DTDs and entity sets form a normative part of this
1100specification. The complete set of DTD files together with an XML
1101declaration and SGML Open Catalog is included in the <a href=
1102"xhtml1.zip">zip file</a> for this specification.</p>
1103
1104<h2><a name="h-A1" id="h-A1">A.1 Document Type
1105Definitions</a></h2>
1106
1107<p>These DTDs approximate the HTML 4.0 DTDs. It is likely that
1108when the DTDs are modularized, a method of DTD construction will
1109be employed that corresponds more closely to HTML 4.0.</p>
1110
1111<ul>
1112<li>
1113<p><a href="DTD/xhtml1-strict.dtd" type="text/plain">
1114XHTML-1.0-Strict</a></p>
1115</li>
1116
1117<li>
1118<p><a href="DTD/xhtml1-transitional.dtd" type="text/plain">
1119XHTML-1.0-Transitional</a></p>
1120</li>
1121
1122<li>
1123<p><a href="DTD/xhtml1-frameset.dtd" type="text/plain">
1124XHTML-1.0-Frameset</a></p>
1125</li>
1126</ul>
1127
1128<h2><a name="h-A2" id="h-A2">A.2 Entity Sets</a></h2>
1129
1130<p>The XHTML entity sets are the same as for HTML 4.0, but have
1131been modified to be valid XML 1.0 entity declarations. Note the
1132entity for the Euro currency sign (<code>&amp;euro;</code> or
1133<code>&amp;#8364;</code> or <code>&amp;#x20AC;</code>) is defined
1134as part of the special characters.</p>
1135
1136<ul>
1137<li>
1138<p><a href="DTD/xhtml-lat1.ent">Latin-1 characters</a></p>
1139</li>
1140
1141<li>
1142<p><a href="DTD/xhtml-special.ent">Special characters</a></p>
1143</li>
1144
1145<li>
1146<p><a href="DTD/xhtml-symbol.ent">Symbols</a></p>
1147</li>
1148</ul>
1149
1150<!--OddPage-->
1151<h1><a name="prohibitions" id="prohibitions">Appendix B. Element
1152Prohibitions</a></h1>
1153
1154<p><b>This appendix is normative.</b></p>
1155
1156<p>The following elements have prohibitions on which elements
1157they can contain (see <a href="#h-4.9">Section 4.9</a>). This
1158prohibition applies to all depths of nesting, i.e. it contains
1159all the descendant elements.</p>
1160
1161<dl><dt><code class="tag">a</code></dt>
1162<dd>
1163cannot contain other <code>a</code> elements.</dd>
1164<dt><code class="tag">pre</code></dt>
1165<dd>cannot contain the <code>img</code>, <code>object</code>,
1166<code>big</code>, <code>small</code>, <code>sub</code>, or <code>
1167sup</code> elements.</dd>
1168
1169<dt><code class="tag">button</code></dt>
1170<dd>cannot contain the <code>input</code>, <code>select</code>,
1171<code>textarea</code>, <code>label</code>, <code>button</code>,
1172<code>form</code>, <code>fieldset</code>, <code>iframe</code> or
1173<code>isindex</code> elements.</dd>
1174<dt><code class="tag">label</code></dt>
1175<dd>cannot contain other <code class="tag">label</code> elements.</dd>
1176<dt><code class="tag">form</code></dt>
1177<dd>cannot contain other <code>form</code> elements.</dd>
1178</dl>
1179
1180<!--OddPage-->
1181<h1><a name="guidelines" id="guidelines">Appendix C.
1182HTML Compatibility Guidelines</a></h1>
1183
1184<p><b>This appendix is informative.</b></p>
1185
1186<p>This appendix summarizes design guidelines for authors who
1187wish their XHTML documents to render on existing HTML user
1188agents.</p>
1189
1190<h2>C.1 Processing Instructions</h2>
1191<p>Be aware that processing instructions are rendered on some
1192user agents. However, also note that when the XML declaration is not included
1193in a document, the document can only use the default character encodings UTF-8
1194or UTF-16.</p>
1195
1196<h2>C.2 Empty Elements</h2>
1197<p>Include a space before the trailing <code>/</code> and <code>
1198&gt;</code> of empty elements, e.g. <code class="greenmono">
1199&lt;br&#160;/&gt;</code>, <code class="greenmono">
1200&lt;hr&#160;/&gt;</code> and <code class="greenmono">&lt;img
1201src="karen.jpg" alt="Karen"&#160;/&gt;</code>. Also, use the
1202minimized tag syntax for empty elements, e.g. <code class=
1203"greenmono">&lt;br /&gt;</code>, as the alternative syntax <code
1204class="greenmono">&lt;br&gt;&lt;/br&gt;</code> allowed by XML
1205gives uncertain results in many existing user agents.</p>
1206
1207<h2>C.3 Element Minimization and Empty Element Content</h2>
1208<p>Given an empty instance of an element whose content model is
1209not <code>EMPTY</code> (for example, an empty title or paragraph)
1210do not use the minimized form (e.g. use <code class="greenmono">
1211&lt;p&gt; &lt;/p&gt;</code> and not <code class="greenmono">
1212&lt;p&#160;/&gt;</code>).</p>
1213
1214<h2>C.4 Embedded Style Sheets and Scripts</h2>
1215<p>Use external style sheets if your style sheet uses <code>
1216&lt;</code> or <code>&amp;</code> or <code>]]&gt;</code> or <code>--</code>. Use
1217external scripts if your script uses <code>&lt;</code> or <code>
1218&amp;</code> or <code>]]&gt;</code> or <code>--</code>. Note that XML parsers
1219are permitted to silently remove the contents of comments. Therefore, the historical
1220practice of "hiding" scripts and style sheets within comments to make the
1221documents backward compatible is likely to not work as expected in XML-based
1222implementations.</p>
1223
1224<h2>C.5 Line Breaks within Attribute Values</h2>
1225<p>Avoid line breaks and multiple whitespace characters within
1226attribute values. These are handled inconsistently by user
1227agents.</p>
1228
1229<h2>C.6 Isindex</h2>
1230<p>Don't include more than one <code>isindex</code> element in
1231the document <code>head</code>. The <code>isindex</code> element
1232is deprecated in favor of the <code>input</code> element.</p>
1233
1234<h2>C.7 The <code>lang</code> and <code>xml:lang</code> Attributes</h2>
1235<p>Use both the <code>lang</code> and <code>xml:lang</code>
1236attributes when specifying the language of an element. The value
1237of the <code>xml:lang</code> attribute takes precedence.</p>
1238
1239<h2>C.8 Fragment Identifiers</h2>
1240<p>In XML, <abbr title="Uniform Resource Identifiers">URIs</abbr> [<a href="#ref-rfc2396">RFC2396</a>] that end with fragment identifiers of the form
1241<code>"#foo"</code> do not refer to elements with an attribute
1242<code>name="foo"</code>; rather, they refer to elements with an
1243attribute defined to be of type <code>ID</code>, e.g., the <code>
1244id</code> attribute in HTML 4.0. Many existing HTML clients don't
1245support the use of <code>ID</code>-type attributes in this way,
1246so identical values may be supplied for both of these attributes to ensure
1247maximum forward and backward compatibility (e.g., <code class=
1248"greenmono">&lt;a id="foo" name="foo"&gt;...&lt;/a&gt;</code>).</p>
1249
1250<p>Further, since the set of
1251legal values for attributes of type <code>ID</code> is much smaller than
1252for those of type <code>CDATA</code>, the type of the <code>name</code>
1253attribute has been changed to <code>NMTOKEN</code>. This attribute is
1254constrained such that it can only have the same values as type
1255<code>ID</code>, or as the <code>Name</code> production in XML 1.0 Section
12562.5, production 5. Unfortunately, this constraint cannot be expressed in the
1257XHTML 1.0 DTDs. Because of this change, care must be taken when
1258converting existing HTML documents. The values of these attributes
1259must be unique within the document, valid, and any references to these
1260fragment identifiers (both
1261internal and external) must be updated should the values be changed during
1262conversion.</p>
1263<p>Finally, note that XHTML 1.0 has deprecated the
1264<code>name</code> attribute of the <code>a</code>, <code>applet</code>, <code>frame</code>, <code>iframe</code>, <code>img</code>, and <code>map</code>
1265elements, and it will be
1266removed from XHTML in subsequent versions.</p>
1267
1268<h2>C.9 Character Encoding</h2>
1269<p>To specify a character encoding in the document, use both the
1270encoding attribute specification on the xml declaration (e.g.
1271<code class="greenmono">&lt;?xml version="1.0"
1272encoding="EUC-JP"?&gt;</code>) and a meta http-equiv statement
1273(e.g. <code class="greenmono">&lt;meta http-equiv="Content-type"
1274content='text/html; charset="EUC-JP"'&#160;/&gt;</code>). The
1275value of the encoding attribute of the xml processing instruction
1276takes precedence.</p>
1277
1278<h2>C.10 Boolean Attributes</h2>
1279<p>Some HTML user agents are unable to interpret boolean
1280attributes when these appear in their full (non-minimized) form,
1281as required by XML 1.0. Note this problem doesn't effect user
1282agents compliant with HTML 4.0. The following attributes are
1283involved: <code>compact</code>, <code>nowrap</code>, <code>
1284ismap</code>, <code>declare</code>, <code>noshade</code>, <code>
1285checked</code>, <code>disabled</code>, <code>readonly</code>,
1286<code>multiple</code>, <code>selected</code>, <code>
1287noresize</code>, <code>defer</code>.</p>
1288
1289<h2>C.11 Document Object Model and XHTML</h2>
1290<p>
1291The Document Object Model level 1 Recommendation [<a href="#ref-dom">DOM</a>]
1292defines document object model interfaces for XML and HTML 4.0. The HTML 4.0
1293document object model specifies that HTML element and attribute names are
1294returned in upper-case. The XML document object model specifies that
1295element and attribute names are returned in the case they are specified. In
1296XHTML 1.0, elements and attributes are specified in lower-case. This apparent difference can be
1297addressed in two ways:
1298</p>
1299<ol>
1300<li>Applications that access XHTML documents served as Internet media type
1301<code>text/html</code>
1302via the <abbr title="Document Object Model">DOM</abbr> can use the HTML DOM,
1303and can rely upon element and attribute names being returned in
1304upper-case from those interfaces.</li>
1305<li>Applications that access XHTML documents served as Internet media types
1306<code>text/xml</code> or <code>application/xml</code>
1307can also use the XML DOM. Elements and attributes will be returned in lower-case.
1308Also, some XHTML elements may or may
1309not appear
1310in the object tree because they are optional in the content model
1311(e.g. the <code>tbody</code> element within
1312<code>table</code>). This occurs because in HTML 4.0 some elements were
1313permitted to be minimized such that their start and end tags are both omitted
1314(an SGML feature).
1315This is not possible in XML. Rather than require document authors to insert
1316extraneous elements, XHTML has made the elements optional.
1317Applications need to adapt to this
1318accordingly.</li>
1319</ol>
1320
1321<h2>C.12 Using Ampersands in Attribute Values</h2>
1322<p>
1323When an attribute value contains an ampersand, it must be expressed as a character
1324entity reference
1325(e.g. "<code>&amp;amp;</code>"). For example, when the
1326<code>href</code> attribute
1327of the <code>a</code> element refers to a
1328CGI script that takes parameters, it must be expressed as
1329<code>http://my.site.dom/cgi-bin/myscript.pl?class=guest&amp;amp;name=user</code>
1330rather than as
1331<code>http://my.site.dom/cgi-bin/myscript.pl?class=guest&amp;name=user</code>.
1332</p>
1333
1334<h2>C.13 Cascading Style Sheets (CSS) and XHTML</h2>
1335
1336<p>The Cascading Style Sheets level 2 Recommendation [<a href="#ref-css2">CSS2</a>] defines style
1337properties which are applied to the parse tree of the HTML or XML
1338document. Differences in parsing will produce different visual or
1339aural results, depending on the selectors used. The following hints
1340will reduce this effect for documents which are served without
1341modification as both media types:</p>
1342
1343<ol>
1344<li>
1345CSS style sheets for XHTML should use lower case element and
1346attribute names.</li>
1347
1348
1349<li>In tables, the tbody element will be inferred by the parser of an
1350HTML user agent, but not by the parser of an XML user agent. Therefore
1351you should always explicitly add a tbody element if it is referred to
1352in a CSS selector.</li>
1353
1354<li>Within the XHTML name space, user agents are expected to
1355recognize the "id" attribute as an attribute of type ID.
1356Therefore, style sheets should be able to continue using the
1357shorthand "#" selector syntax even if the user agent does not read
1358the DTD.</li>
1359
1360<li>Within the XHTML name space, user agents are expected to
1361recognize the "class" attribute. Therefore, style sheets should be
1362able to continue using the shorthand "." selector syntax.</li>
1363
1364<li>
1365CSS defines different conformance rules for HTML and XML documents;
1366be aware that the HTML rules apply to XHTML documents delivered as
1367HTML and the XML rules apply to XHTML documents delivered as XML.</li>
1368</ol>
1369<!--OddPage-->
1370<h1><a name="acks" id="acks">Appendix D.
1371Acknowledgements</a></h1>
1372
1373<p><b>This appendix is informative.</b></p>
1374
1375<p>This specification was written with the participation of the
1376members of the W3C HTML working group:</p>
1377
1378<dl>
1379<dd>Steven Pemberton, CWI (HTML Working Group Chair)<br />
1380Murray Altheim, Sun Microsystems<br />
1381Daniel Austin, CNET: The Computer Network<br />
1382Frank Boumphrey, HTML Writers Guild<br />
1383John Burger, Mitre<br />
1384Andrew W. Donoho, IBM<br />
1385Sam Dooley, IBM<br />
1386Klaus Hofrichter, GMD<br />
1387Philipp Hoschka, W3C<br />
1388Masayasu Ishikawa, W3C<br />
1389Warner ten Kate, Philips Electronics<br />
1390Peter King, Phone.com<br />
1391Paula Klante, JetForm<br />
1392Shin'ichi Matsui, W3C/Panasonic<br />
1393Shane McCarron, Applied Testing and Technology (The Open Group through August
13941999)<br />
1395Ann Navarro, HTML Writers Guild<br />
1396Zach Nies, Quark<br />
1397Dave Raggett, W3C/HP (W3C lead for HTML)<br />
1398Patrick Schmitz, Microsoft<br />
1399Sebastian Schnitzenbaumer, Stack Overflow<br />
1400Chris Wilson, Microsoft<br />
1401Ted Wugofski, Gateway 2000<br />
1402Dan Zigmond, WebTV Networks</dd>
1403</dl>
1404
1405<!--OddPage-->
1406<h1><a name="refs" id="refs">Appendix E. References</a></h1>
1407
1408<p><b>This appendix is informative.</b></p>
1409
1410<dl>
1411
1412<dt><a name="ref-css2" id="ref-css2"><b>[CSS2]</b></a></dt>
1413
1414<dd><a href="http://www.w3.org/TR/REC-CSS2">"Cascading Style Sheets, level 2 (CSS2) Specification"</a>, B.
1415Bos, H. W. Lie, C. Lilley, I. Jacobs, 12 May 1998.<br />
1416Available at: <a href="http://www.w3.org/TR/REC-CSS2">
1417http://www.w3.org/TR/REC-CSS2</a></dd>
1418
1419<dt><a name="ref-dom" id="ref-dom"><b>[DOM]</b></a></dt>
1420
1421<dd><a href="http://www.w3.org/TR/REC-DOM-Level-1">"Document Object Model (DOM) Level 1 Specification"</a>, Lauren
1422Wood <i>et al.</i>, 1 October 1998.<br />
1423Available at: <a href="http://www.w3.org/TR/REC-DOM-Level-1">
1424http://www.w3.org/TR/REC-DOM-Level-1</a></dd>
1425
1426<dt><a name="ref-html4" id="ref-html4"><b>[HTML]</b></a></dt>
1427
1428<dd><a href="http://www.w3.org/TR/1999/PR-html40-19990824">"HTML 4.01 Specification"</a>, D. Raggett, A. Le&#160;Hors, I.
1429Jacobs, 24 August 1999.<br />
1430Available at: <a href="http://www.w3.org/TR/1999/PR-html40-19990824">
1431http://www.w3.org/TR/1999/PR-html40-19990824</a></dd>
1432
1433<dt><a name="ref-posix" id="ref-posix"><b>[POSIX.1]</b></a></dt>
1434
1435<dd>"ISO/IEC 9945-1:1990 Information Technology - Portable
1436Operating System Interface (POSIX) - Part 1: System Application
1437Program Interface (API) [C Language]", Institute of Electrical
1438and Electronics Engineers, Inc, 1990.</dd>
1439
1440<dt><a name="ref-rfc2046" id="ref-rfc2046"><b>
1441[RFC2046]</b></a></dt>
1442
1443<dd><a href="http://www.ietf.org/rfc/rfc2046.txt">"RFC2046: Multipurpose Internet Mail Extensions (MIME) Part
1444Two: Media Types"</a>, N. Freed and N. Borenstein, November
14451996.<br />
1446Available at <a href="http://www.ietf.org/rfc/rfc2046.txt">
1447http://www.ietf.org/rfc/rfc2046.txt</a>. Note that this RFC
1448obsoletes RFC1521, RFC1522, and RFC1590.</dd>
1449
1450<dt><a name="ref-rfc2119" id="ref-rfc2119"><b>
1451[RFC2119]</b></a></dt>
1452
1453<dd><a href="http://www.ietf.org/rfc/rfc2119.txt">"RFC2119: Key words for use in RFCs to Indicate Requirement
1454Levels"</a>, S. Bradner, March 1997.<br />
1455Available at: <a href="http://www.ietf.org/rfc/rfc2119.txt">
1456http://www.ietf.org/rfc/rfc2119.txt</a></dd>
1457
1458<dt><a name="ref-rfc2376" id="ref-rfc2376"><b>
1459[RFC2376]</b></a></dt>
1460
1461<dd><a href="http://www.ietf.org/rfc/rfc2376.txt">"RFC2376: XML Media Types"</a>, E. Whitehead, M. Murata, July
14621998.<br />
1463Available at: <a href="http://www.ietf.org/rfc/rfc2376.txt">
1464http://www.ietf.org/rfc/rfc2376.txt</a></dd>
1465
1466<dt><a name="ref-rfc2396" id="ref-rfc2396"><b>
1467[RFC2396]</b></a></dt>
1468
1469<dd><a href="http://www.ietf.org/rfc/rfc2396.txt">"RFC2396: Uniform Resource Identifiers (URI): Generic
1470Syntax"</a>, T. Berners-Lee, R. Fielding, L. Masinter, August
14711998.<br />
1472This document updates RFC1738 and RFC1808.<br />
1473Available at: <a href="http://www.ietf.org/rfc/rfc2396.txt">
1474http://www.ietf.org/rfc/rfc2396.txt</a></dd>
1475
1476<dt><a name="ref-xml" id="ref-xml"><b>[XML]</b></a></dt>
1477
1478<dd><a href="http://www.w3.org/TR/REC-xml">"Extensible Markup Language (XML) 1.0 Specification"</a>, T.
1479Bray, J. Paoli, C. M. Sperberg-McQueen, 10 February 1998.<br />
1480Available at: <a href="http://www.w3.org/TR/REC-xml">
1481http://www.w3.org/TR/REC-xml</a></dd>
1482
1483<dt><a name="ref-xmlns" id="ref-xmlns"><b>[XMLNAMES]</b></a></dt>
1484
1485<dd><a href="http://www.w3.org/TR/REC-xml-names">"Namespaces in XML"</a>, T. Bray, D. Hollander, A. Layman, 14
1486January 1999.<br />
1487XML namespaces provide a simple method for qualifying names used
1488in XML documents by associating them with namespaces identified
1489by URI.<br />
1490Available at: <a href="http://www.w3.org/TR/REC-xml-names">
1491http://www.w3.org/TR/REC-xml-names</a></dd>
1492
1493</dl>
1494<p><a href="http://www.w3.org/WAI/WCAG1AAA-Conformance"
1495title="Explanation of Level Triple-A Conformance">
1496<img height="32" width="88"
1497src="wcag1AAA.gif"
1498alt="Level Triple-A conformance icon, W3C-WAI Web Content Accessibility Guidelines 1.0" /></a></p>
1499<div class="navbar">
1500 <hr />
1501 <a href="#toc">table of contents</a>
1502</div>
1503</body>
1504</html>
1505
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