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1<?xml version="1.0"?>
2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "dtds/xhtml1-strict.dtd">
3<?xml-stylesheet href="W3C-PR.css" type="text/css"?>
4<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
5<head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
6<title>XHTML 1.0: The Extensible HyperText Markup
7Language</title>
8<link rel="stylesheet" href="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" src="w3c_home.gif" alt="W3C" /></a></p>
60
61<h1 class="head"><a name="title" id="title">XHTML</a><sup>&#x2122;</sup> 1.0:
62The Extensible HyperText Markup Language</h1>
63
64<h2>A Reformulation of HTML 4.0 in XML 1.0</h2>
65
66<h3>W3C Proposed Recommendation 10 December 1999</h3>
67
68<dl>
69<dt>This version:</dt>
70
71<dd><a href="http://www.w3.org/TR/1999/PR-xhtml1-19991210">
72http://www.w3.org/TR/1999/PR-xhtml1-19991210</a> <br />
73(<a href="xhtml1.ps">Postscript version</a>,
74<a href="xhtml1.pdf">PDF version</a>,
75<a href="xhtml1.zip">ZIP archive</a>, or
76<a href="xhtml1.tgz">Gzip'd TAR archive</a>)
77</dd>
78
79<dt>Latest version:</dt>
80
81<dd><a href="http://www.w3.org/TR/xhtml1">
82http://www.w3.org/TR/xhtml1</a></dd>
83
84<dt>Previous versions:</dt>
85
86<dd><a href="http://www.w3.org/TR/1999/WD-xhtml1-19991124">
87http://www.w3.org/TR/1999/WD-xhtml1-19991124</a></dd>
88<dd><a href="http://www.w3.org/TR/1999/PR-xhtml1-19990824">
89http://www.w3.org/TR/1999/PR-xhtml1-19990824</a></dd>
90
91<dt>Authors:</dt>
92
93<dd>See <a href="#acks">acknowledgements</a>.</dd>
94</dl>
95
96<p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">
97Copyright</a> &copy; 1999 <a href="http://www.w3.org/">W3C</a><sup>&reg;</sup>
98(<a href="http://www.lcs.mit.edu/">MIT</a>, <a href="http://www.inria.fr/">INRIA</a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. <abbr title="World Wide Web Consortium">W3C</abbr> <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">
99liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">
100trademark</a>, <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document
101use</a> and <a href="http://www.w3.org/Consortium/Legal/copyright-software">software
102licensing</a> rules apply.</p>
103<hr />
104</div>
105
106<h2 class="notoc">Abstract</h2>
107
108<p>This specification defines <abbr title="Extensible Hypertext Markup Language">XHTML</abbr> 1.0, a reformulation of HTML
1094.0 as an XML 1.0 application, and three <abbr title="Document Type Definition">DTDs</abbr> corresponding to
110the ones defined by HTML 4.0. The semantics of the elements and
111their attributes are defined in the W3C Recommendation for HTML
1124.0. These semantics provide the foundation for future
113extensibility of XHTML. Compatibility with existing HTML user
114agents is possible by following a small set of guidelines.</p>
115
116<h2>Status of this document</h2>
117
118<p><em>This section describes the status of this document at the time
119of its publication. Other documents may supersede this document. The
120latest status of this document series is maintained at the W3C.</em></p>
121
122<p>This specification is a Proposed Recommendation of the HTML Working Group. It is
123a revision of the Proposed Recommendation dated <a href="http://www.w3.org/TR/1999/PR-xhtml1-19990824/">24 August
1241999</a> incorporating changes as a result of comments from the Proposed
125Recommendation review, and
126comments and further deliberations of the W3C HTML Working Group. A
127<a href="xhtml1-diff-19991210.html">diff-marked version</a> from the previous
128proposed recommendation is available for comparison purposes.</p>
129
130<p>On 10 December 1999, this document enters a
131<a href="http://www.w3.org/Consortium/Process/#RecsPR">
132Proposed Recommendation</a> review period. From that date until 8 January
1332000,
134W3C Advisory Committee representatives are encouraged
135to review this specification and return comments in their completed
136ballots to [email protected]. Please send any comments of a
137confidential nature in separate email to [email protected], which is
138visible to the Team only.</p>
139
140<p>No sooner than 14 days after the end of the review period, the
141Director will announce the document's disposition: it may become a W3C
142Recommendation (possibly with minor changes), it may revert to Working
143Draft status, or it may be dropped as a W3C work item.</p>
144
145<p>Publication as a Proposed Recommendation does not imply endorsement
146by the W3C membership. This is still a draft document and may be
147updated, replaced or obsoleted by other documents at any time. It is
148inappropriate to cite W3C Proposed Recommendation as other than "work
149in progress."</p>
150
151<p>This document has been produced as part of the <a href="http://www.w3.org/MarkUp/">W3C HTML Activity</a>. The goals of
152the <a href="http://www.w3.org/MarkUp/Group/">HTML Working
153Group</a> <i>(<a href="http://cgi.w3.org/MemberAccess/">members
154only</a>)</i> are discussed in the <a href="http://www.w3.org/MarkUp/Group/HTMLcharter">HTML Working Group
155charter</a> <i>(<a href="http://cgi.w3.org/MemberAccess/">members
156only</a>)</i>.</p>
157
158<p>A list of current W3C Recommendations and other technical documents
159can be found at <a href="http://www.w3.org/TR">http://www.w3.org/TR</a>.</p>
160
161<p>Public discussion on <abbr title="HyperText Markup Language">HTML</abbr> features takes place on the mailing list <a href="mailto:[email protected]"> [email protected]</a> (<a href="http://lists.w3.org/Archives/Public/www-html/">archive</a>). The W3C
162staff contact for work on HTML is <a href="mailto:[email protected]">Dave
163Raggett</a>.</p>
164
165<p>Please report errors in this document to <a href="mailto:[email protected]">[email protected]</a>.</p>
166
167<p>The list of known errors in this specification is available at <a href="http://www.w3.org/1999/12/PR-xhtml1-19991210-errata">http://www.w3.org/1999/12/PR-xhtml1-19991210-errata</a>.</p>
168
169<h2 class="notoc"><a id="toc" name="toc">Contents</a></h2>
170
171<div class="contents">
172<ul class="toc">
173<li class="tocline">1. <a href="#xhtml">What is XHTML?</a>
174
175<ul class="toc">
176<li class="tocline">1.1 <a href="#html4">What is HTML 4.0?</a></li>
177
178<li class="tocline">1.2 <a href="#xml">What is XML?</a></li>
179
180<li class="tocline">1.3 <a href="#why">Why the need for XHTML?</a></li>
181</ul>
182</li>
183
184<li class="tocline">2. <a href="#defs">Definitions</a>
185
186<ul class="toc">
187<li class="tocline">2.1 <a href="#terms">Terminology</a></li>
188
189<li class="tocline">2.2 <a href="#general">General Terms</a></li>
190</ul>
191</li>
192
193<li class="tocline">3. <a href="#normative">Normative Definition of XHTML 1.0</a>
194
195
196<ul class="toc">
197<li class="tocline">3.1 <a href="#docconf">Document Conformance</a></li>
198
199<li class="tocline">3.2 <a href="#uaconf">User Agent Conformance</a></li>
200</ul>
201</li>
202
203<li class="tocline">4. <a href="#diffs">Differences with HTML 4.0</a>
204
205</li>
206
207<li class="tocline">5. <a href="#issues">Compatibility Issues</a>
208
209<ul class="toc">
210<li class="tocline">5.1 <a href="#media">Internet Media Types</a></li>
211</ul>
212</li>
213
214<li class="tocline">6. <a href="#future">Future Directions</a>
215
216<ul class="toc">
217<li class="tocline">6.1 <a href="#mods">Modularizing HTML</a></li>
218
219<li class="tocline">6.2 <a href="#extensions">Subsets and Extensibility</a></li>
220
221<li class="tocline">6.3 <a href="#profiles">Document Profiles</a></li>
222</ul>
223</li>
224
225<li class="tocline"><a href="#dtds">Appendix A. DTDs</a></li>
226
227<li class="tocline"><a href="#prohibitions">Appendix B. Element
228Prohibitions</a></li>
229
230<li class="tocline"><a href="#guidelines">Appendix C. HTML Compatibility Guidelines</a></li>
231
232<li class="tocline"><a href="#acks">Appendix D. Acknowledgements</a></li>
233
234<li class="tocline"><a href="#refs">Appendix E. References</a></li>
235</ul>
236</div>
237
238<!--OddPage-->
239<h1><a name="xhtml" id="xhtml">1. What is XHTML?</a></h1>
240
241<p>XHTML is a family of current and future document types and modules that
242reproduce, 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,
243and ultimately are designed to work in conjunction with XML-based user agents.
244The details of this family and its evolution are
245discussed in more detail in the section on <a href="#future">Future
246Directions</a>. </p>
247
248<p>XHTML 1.0 (this specification) is the first document type in the XHTML
249family. It is a reformulation of the three HTML 4.0 document types as
250applications of XML 1.0 <a href="#ref-xml"> [XML]</a>. It is intended
251to be used as a language for content that is both XML-conforming and, if some
252simple <a href="#guidelines">guidelines</a> are followed,
253operates in HTML 4.0 conforming user agents. Developers who migrate
254their content to XHTML 1.0 will realize the following benefits:</p>
255
256<ul>
257<li>XHTML documents are XML conforming. As such, they are readily viewed,
258edited, and validated with standard XML tools.</li>
259<li>XHTML documents can be written to
260to operate as well or better than they did before in existing
261HTML 4.0-conforming user agents as well as in new, XHTML 1.0 conforming user
262agents.</li>
263<li>XHTML documents can utilize applications (e.g. scripts and applets) that rely
264upon either the HTML Document Object Model or the XML Document Object Model <a href="#ref-dom">[DOM]</a>.</li>
265<li>As the XHTML family evolves, documents conforming to XHTML 1.0 will be more
266likely to interoperate within and among various XHTML environments.</li>
267</ul>
268
269<p>The XHTML family is the next step in the evolution of the Internet. By
270migrating to XHTML today, content developers can enter the XML world with all
271of its attendant benefits, while still remaining confident in their
272content's backward and future compatibility.</p>
273
274<h2><a name="html4" id="html4">1.1 What is HTML 4.0?</a></h2>
275
276<p>HTML 4.0 <a href="#ref-html4">[HTML]</a> is an <abbr title="Standard Generalized Markup Language">SGML</abbr> (Standard
277Generalized Markup Language) application conforming to
278International Standard <abbr title="Organization for International Standardization">ISO</abbr> 8879, and is widely regarded as the
279standard publishing language of the World Wide Web.</p>
280
281<p>SGML is a language for describing markup languages,
282particularly those used in electronic document exchange, document
283management, and document publishing. HTML is an example of a
284language defined in SGML.</p>
285
286<p>SGML has been around since the middle 1980's and has remained
287quite stable. Much of this stability stems from the fact that the
288language is both feature-rich and flexible. This flexibility,
289however, comes at a price, and that price is a level of
290complexity that has inhibited its adoption in a diversity of
291environments, including the World Wide Web.</p>
292
293<p>HTML, as originally conceived, was to be a language for the
294exchange of scientific and other technical documents, suitable
295for use by non-document specialists. HTML addressed the problem
296of SGML complexity by specifying a small set of structural and
297semantic tags suitable for authoring relatively simple documents.
298In addition to simplifying the document structure, HTML added
299support for hypertext. Multimedia capabilities were added
300later.</p>
301
302<p>In a remarkably short space of time, HTML became wildly
303popular and rapidly outgrew its original purpose. Since HTML's
304inception, there has been rapid invention of new elements for use
305within HTML (as a standard) and for adapting HTML to vertical,
306highly specialized, markets. This plethora of new elements has
307led to compatibility problems for documents across different
308platforms.</p>
309
310<p>As the heterogeneity of both software and platforms rapidly
311proliferate, it is clear that the suitability of 'classic' HTML
3124.0 for use on these platforms is somewhat limited.</p>
313
314<h2><a name="xml" id="xml">1.2 What is XML?</a></h2>
315
316<p>XML<sup>&#x2122;</sup> is the shorthand for Extensible Markup
317Language, and is an acronym of Extensible Markup Language <a href="#ref-xml">[XML]</a>.</p>
318
319<p>XML was conceived as a means of regaining the power and
320flexibility of SGML without most of its complexity. Although a
321restricted form of SGML, XML nonetheless preserves most of SGML's
322power and richness, and yet still retains all of SGML's commonly
323used features.</p>
324
325<p>While retaining these beneficial features, XML removes many of
326the more complex features of SGML that make the authoring and
327design of suitable software both difficult and costly.</p>
328
329<h2><a name="why" id="why">1.3 Why the need for XHTML?</a></h2>
330
331<p>The benefits of migrating to XHTML 1.0 are described above. Some of the
332benefits of migrating to XHTML in general are:</p>
333
334<ul>
335<li>Document developers and user agent designers are constantly
336discovering new ways to express their ideas through new markup. In XML, it is
337relatively easy to introduce new elements or additional element
338attributes. The XHTML family is designed to accommodate these extensions
339through XHTML modules and techniques for developing new XHTML-conforming
340modules (described in the forthcoming XHTML Modularization specification).
341These modules will permit the combination of existing and
342new feature sets when developing content and when designing new user
343agents.</li>
344
345<li>Alternate ways of accessing the Internet are constantly being
346introduced. Some estimates indicate that by the year 2002, 75% of
347Internet document viewing will be carried out on these alternate
348platforms. The XHTML family is designed with general user agent
349interoperability in mind. Through a new user agent and document profiling
350mechanism, servers, proxies, and user agents will be able to perform
351best effort content transformation. Ultimately, it will be possible to
352develop XHTML-conforming content that is usable by any XHTML-conforming
353user agent.</li>
354
355</ul>
356<!--OddPage-->
357<h1><a name="defs" id="defs">2. Definitions</a></h1>
358
359<h2><a name="terms" id="terms">2.1 Terminology</a></h2>
360
361<p>The following terms are used in this specification. These
362terms extend the definitions in <a href="#ref-rfc2119">
363[RFC2119]</a> in ways based upon similar definitions in ISO/<abbr title="International Electro-technical Commission">IEC</abbr>
3649945-1:1990 <a href="#ref-posix">[POSIX.1]</a>:</p>
365
366<dl>
367<dt>Implementation-defined</dt>
368
369<dd>A value or behavior is implementation-defined when it is left
370to the implementation to define [and document] the corresponding
371requirements for correct document construction.</dd>
372
373<dt>May</dt>
374
375<dd>With respect to implementations, the word "may" is to be
376interpreted as an optional feature that is not required in this
377specification but can be provided. With respect to <a href="#docconf">Document Conformance</a>, the word "may" means that
378the optional feature must not be used. The term "optional" has
379the same definition as "may".</dd>
380
381<dt>Must</dt>
382
383<dd>In this specification, the word "must" is to be interpreted
384as a mandatory requirement on the implementation or on Strictly
385Conforming XHTML Documents, depending upon the context. The term
386"shall" has the same definition as "must".</dd>
387
388<dt>Reserved</dt>
389
390<dd>A value or behavior is unspecified, but it is not allowed to
391be used by Conforming Documents nor to be supported by a
392Conforming User Agents.</dd>
393
394<dt>Should</dt>
395
396<dd>With respect to implementations, the word "should" is to be
397interpreted as an implementation recommendation, but not a
398requirement. With respect to documents, the word "should" is to
399be interpreted as recommended programming practice for documents
400and a requirement for Strictly Conforming XHTML Documents.</dd>
401
402<dt>Supported</dt>
403
404<dd>Certain facilities in this specification are optional. If a
405facility is supported, it behaves as specified by this
406specification.</dd>
407
408<dt>Unspecified</dt>
409
410<dd>When a value or behavior is unspecified, the specification
411defines no portability requirements for a facility on an
412implementation even when faced with a document that uses the
413facility. A document that requires specific behavior in such an
414instance, rather than tolerating any behavior when using that
415facility, is not a Strictly Conforming XHTML Document.</dd>
416</dl>
417
418<h2><a name="general" id="general">2.2 General Terms</a></h2>
419
420<dl>
421<dt>Attribute</dt>
422
423<dd>An attribute is a parameter to an element declared in the
424DTD. An attribute's type and value range, including a possible
425default value, are defined in the DTD.</dd>
426
427<dt>DTD</dt>
428
429<dd>A DTD, or document type definition, is a collection of XML
430declarations that, as a collection, defines the legal structure,
431<span class="term">elements</span>, and <span class="term">
432attributes</span> that are available for use in a document that
433complies to the DTD.</dd>
434
435<dt>Document</dt>
436
437<dd>A document is a stream of data that, after being combined
438with any other streams it references, is structured such that it
439holds information contained within <span class="term">
440elements</span> that are organized as defined in the associated
441<span class="term">DTD</span>. See <a href="#docconf">Document
442Conformance</a> for more information.</dd>
443
444<dt>Element</dt>
445
446<dd>An element is a document structuring unit declared in the
447<span class="term">DTD</span>. The element's content model is
448defined in the <span class="term">DTD</span>, and additional
449semantics may be defined in the prose description of the
450element.</dd>
451
452<dt><a name="facilities" id="facilities">Facilities</a></dt>
453
454<dd>Functionality includes <span class="term">elements</span>,
455<span class="term">attributes</span>, and the semantics
456associated with those <span class="term">elements</span> and
457<span class="term">attributes</span>. An implementation
458supporting that functionality is said to provide the necessary
459facilities.</dd>
460
461<dt>Implementation</dt>
462
463<dd>An implementation is a system that provides collection of
464<span class="term">facilities</span> and services that supports
465this specification. See <a href="#uaconf">User Agent
466Conformance</a> for more information.</dd>
467
468<dt>Parsing</dt>
469
470<dd>Parsing is the act whereby a <span class="term">
471document</span> is scanned, and the information contained within
472the <span class="term">document</span> is filtered into the
473context of the <span class="term">elements</span> in which the
474information is structured.</dd>
475
476<dt>Rendering</dt>
477
478<dd>Rendering is the act whereby the information in a <span class="term">document</span> is presented. This presentation is
479done in the form most appropriate to the environment (e.g.
480aurally, visually, in print).</dd>
481
482<dt>User Agent</dt>
483
484<dd>A user agent is an <span class="term">implementation</span>
485that retrieves and processes XHTML documents. See <a href="#uaconf">User Agent Conformance</a> for more information.</dd>
486
487<dt>Validation</dt>
488
489<dd>Validation is a process whereby <span class="term">
490documents</span> are verified against the associated <span class="term">DTD</span>, ensuring that the structure, use of <span class="term">elements</span>, and use of <span class="term">
491attributes</span> are consistent with the definitions in the
492<span class="term">DTD</span>.</dd>
493
494<dt><a name="wellformed" id="wellformed">Well-formed</a></dt>
495
496<dd>A <span class="term">document</span> is well-formed when it
497is structured according to the rules defined in <a href="http://www.w3.org/TR/REC-xml#sec-well-formed">Section 2.1</a> of
498the XML 1.0 Recommendation <a href="#ref-xml">[XML]</a>.
499Basically, this definition states that elements, delimited by
500their start and end tags, are nested properly within one
501another.</dd>
502</dl>
503
504<!--OddPage-->
505<h1><a name="normative" id="normative">3. Normative Definition of
506XHTML 1.0</a></h1>
507
508<h2><a name="docconf" id="docconf">3.1 Document
509Conformance</a></h2>
510
511<p>This version of XHTML provides a definition of strictly
512conforming XHTML documents, which are restricted to tags and
513attributes from the XHTML namespace. See <a href="#well-formed">Section 3.1.2</a> for information on using XHTML
514with other namespaces, for instance, to include metadata
515expressed in <abbr title="Resource Description Format">RDF</abbr> within XHTML documents.</p>
516
517<h3><a name="strict" id="strict">3.1.1 Strictly Conforming
518Documents</a></h3>
519
520<p>A Strictly Conforming XHTML Document is a document that
521requires only the facilities described as mandatory in this
522specification. Such a document must meet all of the following
523criteria:</p>
524
525<ol>
526<li>
527<p>It must validate against one of the three DTDs found in <a href="#dtds">Appendix&#xA0;A</a>.</p>
528</li>
529
530<li>
531<p>The root element of the document must be <code>
532&lt;html&gt;</code>.</p>
533</li>
534
535<li>
536<p>The root element of the document must designate the XHTML
537namespace using the <code>xmlns</code> attribute <a href="#ref-xmlns">[XMLNAMES]</a>. The namespace for XHTML is
538defined to be
539<code>http://www.w3.org/1999/xhtml</code>.</p>
540</li>
541
542<li>
543<p>There must be a DOCTYPE declaration in the document prior to
544the root element. The public identifier included in
545the DOCTYPE declaration must reference one of the three DTDs
546found in <a href="#dtds">Appendix&#xA0;A</a> using the respective
547Formal Public Identifier. The system identifier may be changed to reflect
548local system conventions.</p>
549
550<pre>
551&lt;!DOCTYPE html
552 PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
553 "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-strict.dtd&gt;
554
555&lt;!DOCTYPE html
556 PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
557 "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-transitional.dtd&gt;
558
559&lt;!DOCTYPE html
560 PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
561 "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-frameset.dtd&gt;
562</pre>
563</li>
564</ol>
565
566<p>Here is an example of a minimal XHTML document.</p>
567
568<div class="good">
569<pre>
570&lt;?xml version="1.0" encoding="UTF-8"?&gt;
571&lt;!DOCTYPE html
572 PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
573 "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-strict.dtd"&gt;
574&lt;html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"&gt;
575 &lt;head&gt;
576 &lt;title&gt;Virtual Library&lt;/title&gt;
577 &lt;/head&gt;
578 &lt;body&gt;
579 &lt;p&gt;Moved to &lt;a href="http://vlib.org/"&gt;vlib.org&lt;/a&gt;.&lt;/p&gt;
580 &lt;/body&gt;
581&lt;/html&gt;</pre>
582</div>
583
584<p>Note that in this example, the XML declaration is included. An XML
585declaration like the one above is
586not required in all XML documents. XHTML document authors are strongly encouraged to use XML declarations in all their documents. Such a declaration is required
587when the character encoding of the document is other than the default UTF-8 or
588UTF-16.</p>
589
590<h3><a name="well-formed" id="well-formed">3.1.2 Using XHTML with
591other namespaces</a></h3>
592
593<p>The XHTML namespace may be used with other XML namespaces
594as per <a href="#ref-xmlns">[XMLNAMES]</a>, although such
595documents are not strictly conforming XHTML 1.0 documents as
596defined above. Future work by W3C will address ways to specify
597conformance for documents involving multiple namespaces.</p>
598
599<p>The following example shows the way in which XHTML 1.0 could
600be used in conjunction with the MathML Recommendation:</p>
601
602<div class="good">
603<pre>
604&lt;html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"&gt;
605 &lt;head&gt;
606 &lt;title&gt;A Math Example&lt;/title&gt;
607 &lt;/head&gt;
608 &lt;body&gt;
609 &lt;p&gt;The following is MathML markup:&lt;/p&gt;
610 &lt;math xmlns="http://www.w3.org/1998/Math/MathML"&gt;
611 &lt;apply&gt; &lt;log/&gt;
612 &lt;logbase&gt;
613 &lt;cn&gt; 3 &lt;/cn&gt;
614 &lt;/logbase&gt;
615 &lt;ci&gt; x &lt;/ci&gt;
616 &lt;/apply&gt;
617 &lt;/math&gt;
618 &lt;/body&gt;
619&lt;/html&gt;
620</pre>
621</div>
622
623<p>The following example shows the way in which XHTML 1.0 markup
624could be incorporated into another XML namespace:</p>
625
626<div class="good">
627<pre>
628&lt;?xml version="1.0" encoding="UTF-8"?&gt;
629&lt;!-- initially, the default namespace is "books" --&gt;
630&lt;book xmlns='urn:loc.gov:books'
631 xmlns:isbn='urn:ISBN:0-395-36341-6' xml:lang="en" lang="en"&gt;
632 &lt;title&gt;Cheaper by the Dozen&lt;/title&gt;
633 &lt;isbn:number&gt;1568491379&lt;/isbn:number&gt;
634 &lt;notes&gt;
635 &lt;!-- make HTML the default namespace for a hypertext commentary --&gt;
636 &lt;p xmlns='http://www.w3.org/1999/xhtml'&gt;
637 This is also available &lt;a href="http://www.w3.org/"&gt;online&lt;/a&gt;.
638 &lt;/p&gt;
639 &lt;/notes&gt;
640&lt;/book&gt;
641</pre>
642</div>
643
644<h2><a name="uaconf" id="uaconf">3.2 User Agent
645Conformance</a></h2>
646
647<p>A conforming user agent must meet all of the following
648criteria:</p>
649
650<ol>
651<li>In order to be consistent with the XML 1.0 Recommendation <a href="#ref-xml">[XML]</a>, the user agent must parse and evaluate
652an XHTML document for well-formedness. If the user agent claims
653to be a validating user agent, it must also validate documents
654against their referenced DTDs according to <a href="#ref-xml">
655[XML]</a>.</li>
656
657<li>When the user agent claims to support <a href="#facilities">
658facilities</a> defined within this specification or required by
659this specification through normative reference, it must do so in
660ways consistent with the facilities' definition.</li>
661
662<li>When a user agent processes an XHTML document as generic XML,
663it shall only recognize attributes of type
664<code>ID</code> (e.g. the <code>id</code> attribute on most XHTML elements)
665as fragment identifiers.</li>
666
667<li>If a user agent encounters an element it does not recognize,
668it must render the element's content.</li>
669
670<li>If a user agent encounters an attribute it does not
671recognize, it must ignore the entire attribute specification
672(i.e., the attribute and its value).</li>
673
674<li>If a user agent encounters an attribute value it doesn't
675recognize, it must use the default attribute value.</li>
676
677<li>If it encounters an entity reference (other than one
678of the predefined entities) for which the User Agent has
679processed no declaration (which could happen if the declaration
680is in the external subset which the User Agent hasn't read), the entity
681reference should be rendered as the characters (starting
682with the ampersand and ending with the semi-colon) that
683make up the entity reference.</li>
684
685<li>When rendering content, User Agents that encounter
686characters 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>
687
688<li>
689The following characters are defined in [XML] as whitespace characters:
690
691<ul>
692<li>Space (&amp;#x0020;)</li>
693<li>Tab (&amp;#x0009;)</li>
694<li>Carriage return (&amp;#x000D;)</li>
695<li>Line feed (&amp;#x000A;)</li>
696</ul>
697
698<p>
699The XML processor normalizes different system's line end codes into one
700single line-feed character, that is passed up to the application. The XHTML
701user agent in addition, must treat the following characters as whitespace:
702</p>
703
704<ul>
705<li>Form feed (&amp;#x000C;)</li>
706<li>Zero-width space (&amp;#x200B;)</li>
707</ul>
708
709<p>
710In elements where the 'xml:space' attribute is set to 'preserve', the user
711agent must leave all whitespace characters intact (with the exception of
712leading and trailing whitespace characters, which should be removed).
713Otherwise, whitespace
714is handled according to the following rules:
715</p>
716
717<ul>
718<li>
719All whitespace surrounding block elements should be removed.
720</li>
721<li>
722Comments are removed entirely and do not affect whitespace handling. One
723whitespace character on either side of a comment is treated as two white
724space characters.
725</li>
726<li>
727Leading and trailing whitespace inside a block element must be removed.
728</li>
729<li>Line feed characters within a block element must be converted into a
730space (except when the 'xml:space' attribute is set to 'preserve').
731</li>
732<li>
733A sequence of white space characters must be reduced to a single space
734character (except when the 'xml:space' attribute is set to 'preserve').
735</li>
736<li>
737With regard to rendition,
738the User Agent should render the content in a
739manner appropriate to the language in which the content is written.
740In languages whose primary script is Latinate, the ASCII space
741character is typically used to encode both grammatical word boundaries and
742typographic whitespace; in languages whose script is related to Nagari
743(e.g., Sanskrit, Thai, etc.), grammatical boundaries may be encoded using
744the ZW 'space' character, but will not typically be represented by
745typographic whitespace in rendered output; languages using Arabiform scripts
746may encode typographic whitespace using a space character, but may also use
747the ZW space character to delimit 'internal' grammatical boundaries (what
748look like words in Arabic to an English eye frequently encode several words,
749e.g. 'kitAbuhum' = 'kitAbu-hum' = 'book them' == their book); and languages
750in the Chinese script tradition typically neither encode such delimiters nor
751use typographic whitespace in this way.
752</li>
753</ul>
754
755<p>Whitespace in attribute values is processed according to <a href="#ref-xml">[XML]</a>.</p>
756</li>
757</ol>
758
759<!--OddPage-->
760<h1><a name="diffs" id="diffs">4. Differences with HTML
7614.0</a></h1>
762
763<p>Due to the fact that XHTML is an XML application, certain
764practices that were perfectly legal in SGML-based HTML 4.0 <a href="#ref-html4">[HTML]</a> must be changed.</p>
765
766<h2><a name="h-4.1" id="h-4.1">4.1 Documents must be
767well-formed</a></h2>
768
769<p><a href="#wellformed">Well-formedness</a> is a new concept
770introduced by <a href="#ref-xml">[XML]</a>. Essentially this
771means that all elements must either have closing tags or be
772written in a special form (as described below), and that all the
773elements must nest.</p>
774
775<p>Although overlapping is illegal in SGML, it was widely
776tolerated in existing browsers.</p>
777
778<div class="good">
779<p><strong><em>CORRECT: nested elements.</em></strong></p>
780
781<p>&lt;p&gt;here is an emphasized
782&lt;em&gt;paragraph&lt;/em&gt;.&lt;/p&gt;</p>
783</div>
784
785<div class="bad">
786<p><strong><em>INCORRECT: overlapping elements</em></strong></p>
787
788<p>&lt;p&gt;here is an emphasized
789&lt;em&gt;paragraph.&lt;/p&gt;&lt;/em&gt;</p>
790</div>
791
792<h2><a name="h-4.2" id="h-4.2">4.2 Element and attribute
793names must be in lower case</a></h2>
794
795<p>XHTML documents must use lower case for all HTML element and
796attribute names. This difference is necessary because XML is
797case-sensitive e.g. &lt;li&gt; and &lt;LI&gt; are different
798tags.</p>
799
800<h2><a name="h-4.3" id="h-4.3">4.3 For non-empty elements,
801end tags are required</a></h2>
802
803<p>In SGML-based HTML 4.0 certain elements were permitted to omit
804the end tag; with the elements that followed implying closure.
805This omission is not permitted in XML-based XHTML. All elements
806other than those declared in the DTD as <code>EMPTY</code> must
807have an end tag.</p>
808
809<div class="good">
810<p><strong><em>CORRECT: terminated elements</em></strong></p>
811
812<p>&lt;p&gt;here is a paragraph.&lt;/p&gt;&lt;p&gt;here is
813another paragraph.&lt;/p&gt;</p>
814</div>
815
816<div class="bad">
817<p><strong><em>INCORRECT: unterminated elements</em></strong></p>
818
819<p>&lt;p&gt;here is a paragraph.&lt;p&gt;here is another
820paragraph.</p>
821</div>
822
823<h2><a name="h-4.4" id="h-4.4">4.4 Attribute values must
824always be quoted</a></h2>
825
826<p>All attribute values must be quoted, even those which appear
827to be numeric.</p>
828
829<div class="good">
830<p><strong><em>CORRECT: quoted attribute values</em></strong></p>
831
832<p>&lt;table rows="3"&gt;</p>
833</div>
834
835<div class="bad">
836<p><strong><em>INCORRECT: unquoted attribute values</em></strong></p>
837
838<p>&lt;table rows=3&gt;</p>
839</div>
840
841<h2><a name="h-4.5" id="h-4.5">4.5 Attribute
842Minimization</a></h2>
843
844<p>XML does not support attribute minimization. Attribute-value
845pairs must be written in full. Attribute names such as <code>
846compact</code> and <code>checked</code> cannot occur in elements
847without their value being specified.</p>
848
849<div class="good">
850<p><strong><em>CORRECT: unminimized attributes</em></strong></p>
851
852<p>&lt;dl compact="compact"&gt;</p>
853</div>
854
855<div class="bad">
856<p><strong><em>INCORRECT: minimized attributes</em></strong></p>
857
858<p>&lt;dl compact&gt;</p>
859</div>
860
861<h2><a name="h-4.6" id="h-4.6">4.6 Empty Elements</a></h2>
862
863<p>Empty elements must either have an end tag or the start tag must end with <code>/&gt;</code>. For instance,
864<code>&lt;br/&gt;</code> or <code>&lt;hr&gt;&lt;/hr&gt;</code>. See <a href="#guidelines">HTML Compatibility Guidelines</a> for information on ways to
865ensure this is backward compatible with HTML 4.0 user agents.</p>
866
867<div class="good">
868<p><strong><em>CORRECT: terminated empty tags</em></strong></p>
869
870<p>&lt;br/&gt;&lt;hr/&gt;</p>
871</div>
872
873<div class="bad">
874<p><strong><em>INCORRECT: unterminated empty tags</em></strong></p>
875
876<p>&lt;br&gt;&lt;hr&gt;</p>
877</div>
878
879<h2><a name="h-4.7" id="h-4.7">4.7 Whitespace handling in
880attribute values</a></h2>
881
882<p>In attribute values, user agents will strip leading and
883trailing whitespace from attribute values and map sequences
884of one or more whitespace characters (including line breaks) to
885a single inter-word space (an ASCII space character for western
886scripts). See <a href="http://www.w3.org/TR/REC-xml#AVNormalize">
887Section 3.3.3</a> of <a href="#ref-xml">[XML]</a>.</p>
888
889<h2><a name="h-4.8" id="h-4.8">4.8 Script and Style
890elements</a></h2>
891
892<p>In XHTML, the script and style elements are declared as having
893<code>#PCDATA</code> content. As a result, <code>&lt;</code> and
894<code>&amp;</code> will be treated as the start of markup, and
895entities such as <code>&amp;lt;</code> and <code>&amp;amp;</code>
896will be recognized as entity references by the XML processor to
897<code>&lt;</code> and <code>&amp;</code> respectively. Wrapping
898the content of the script or style element within a <code>
899CDATA</code> marked section avoids the expansion of these
900entities.</p>
901
902<div class="good">
903<pre>
904&lt;script&gt;
905 &lt;![CDATA[
906 ... unescaped script content ...
907 ]]&gt;
908 &lt;/script&gt;
909</pre>
910</div>
911
912<p><code>CDATA</code> sections are recognized by the XML
913processor and appear as nodes in the Document Object Model, see
914<a href="http://www.w3.org/TR/REC-DOM-Level-1/level-one-core.html#ID-E067D597">
915Section 1.3</a> of the DOM Level 1 Recommendation <a href="#ref-dom">[DOM]</a>.</p>
916
917<p>An alternative is to use external script and style
918documents.</p>
919
920<h2><a name="h-4.9" id="h-4.9">4.9 SGML exclusions</a></h2>
921
922<p>SGML gives the writer of a DTD the ability to exclude specific
923elements from being contained within an element. Such
924prohibitions (called "exclusions") are not possible in XML.</p>
925
926<p>For example, the HTML 4.0 Strict DTD forbids the nesting of an
927'<code>a</code>' element within another '<code>a</code>' element
928to any descendant depth. It is not possible to spell out such
929prohibitions in XML. Even though these prohibitions cannot be
930defined in the DTD, certain elements should not be nested. A
931summary of such elements and the elements that should not be
932nested in them is found in the normative <a href="#prohibitions">
933Appendix&#xA0;B</a>.</p>
934
935<h2><a name="h-4.10" id="h-4.10">4.10 The elements with 'id' and 'name'
936attributes</a></h2>
937
938<p>HTML 4.0 defined the <code>name</code> attribute for the elements
939<code>a</code>,
940<code>applet</code>, <code>frame</code>,
941<code>iframe</code>, <code>img</code>, and <code>map</code>.
942HTML 4.0 also introduced
943the <code>id</code> attribute. Both of these attributes are designed to be
944used as fragment identifiers.</p>
945<p>In XML, fragment identifiers are of type <code>ID</code>, and
946there can only be a single attribute of type <code>ID</code> per element.
947Therefore, in XHTML 1.0 the <code>id</code>
948attribute is defined to be of type <code>ID</code>. In order to
949ensure that XHTML 1.0 documents are well-structured XML documents, XHTML 1.0
950documents MUST use the <code>id</code> attribute when defining fragment
951identifiers, even on elements that historically have also had a
952<code>name</code> attribute.
953See the <a href="#guidelines">HTML Compatibility
954Guidelines</a> for information on ensuring such anchors are backwards
955compatible when serving XHTML documents as media type <code>text/html</code>.
956</p>
957<p>Note that in XHTML 1.0, the <code>name</code> attribute of these
958elements is formally deprecated, and will be removed in a
959subsequent version of XHTML.</p>
960
961<!--OddPage-->
962<h1><a name="issues" id="issues">5. Compatibility Issues</a></h1>
963
964<p>Although there is no requirement for XHTML 1.0 documents to be
965compatible with existing user agents, in practice this is easy to
966accomplish. Guidelines for creating compatible documents can be
967found in <a href="#guidelines">Appendix&#xA0;C</a>.</p>
968
969<h2><a name="media" id="media">5.1 Internet Media Type</a></h2>
970<p>As of the publication of this recommendation, the general
971recommended MIME labeling for XML-based applications
972has yet to be resolved.</p>
973
974<p>However, XHTML Documents which follow the guidelines set forth
975in <a href="#guidelines">Appendix C</a>, "HTML Compatibility Guidelines" may be
976labeled with the Internet Media Type "text/html", as they
977are compatible with most HTML browsers. This document
978makes no recommendation about MIME labeling of other
979XHTML documents.</p>
980
981<!--OddPage-->
982<h1><a name="future" id="future">6. Future Directions</a></h1>
983
984<p>XHTML 1.0 provides the basis for a family of document types
985that will extend and subset XHTML, in order to support a wide
986range of new devices and applications, by defining modules and
987specifying a mechanism for combining these modules. This
988mechanism will enable the extension and sub-setting of XHTML 1.0
989in a uniform way through the definition of new modules.</p>
990
991<h2><a name="mods" id="mods">6.1 Modularizing HTML</a></h2>
992
993<p>As the use of XHTML moves from the traditional desktop user
994agents to other platforms, it is clear that not all of the XHTML
995elements will be required on all platforms. For example a hand
996held device or a cell-phone may only support a subset of XHTML
997elements.</p>
998
999<p>The process of modularization breaks XHTML up into a series of
1000smaller element sets. These elements can then be recombined to
1001meet the needs of different communities.</p>
1002
1003<p>These modules will be defined in a later W3C document.</p>
1004
1005<h2><a name="extensions" id="extensions">6.2 Subsets and
1006Extensibility</a></h2>
1007
1008<p>Modularization brings with it several advantages:</p>
1009
1010<ul>
1011<li>
1012<p>It provides a formal mechanism for sub-setting XHTML.</p>
1013</li>
1014
1015<li>
1016<p>It provides a formal mechanism for extending XHTML.</p>
1017</li>
1018
1019<li>
1020<p>It simplifies the transformation between document types.</p>
1021</li>
1022
1023<li>
1024<p>It promotes the reuse of modules in new document types.</p>
1025</li>
1026</ul>
1027
1028<h2><a name="profiles" id="profiles">6.3 Document
1029Profiles</a></h2>
1030
1031<p>A document profile specifies the syntax and semantics of a set
1032of documents. Conformance to a document profile provides a basis
1033for interoperability guarantees. The document profile specifies
1034the facilities required to process documents of that type, e.g.
1035which image formats can be used, levels of scripting, style sheet
1036support, and so on.</p>
1037
1038<p>For product designers this enables various groups to define
1039their own standard profile.</p>
1040
1041<p>For authors this will obviate the need to write several
1042different versions of documents for different clients.</p>
1043
1044<p>For special groups such as chemists, medical doctors, or
1045mathematicians this allows a special profile to be built using
1046standard HTML elements plus a group of elements geared to the
1047specialist's needs.</p>
1048
1049<!--OddPage-->
1050<h1><a name="appendices" id="appendices"></a>
1051<a name="dtds" id="dtds">Appendix A. DTDs</a></h1>
1052
1053<p><b>This appendix is normative.</b></p>
1054
1055<p>These DTDs and entity sets form a normative part of this
1056specification. The complete set of DTD files together with an XML
1057declaration and SGML Open Catalog is included in the <a href="xhtml1.zip">zip file</a> for this specification.</p>
1058
1059<h2><a name="h-A1" id="h-A1">A.1 Document Type
1060Definitions</a></h2>
1061
1062<p>These DTDs approximate the HTML 4.0 DTDs. It is likely that
1063when the DTDs are modularized, a method of DTD construction will
1064be employed that corresponds more closely to HTML 4.0.</p>
1065
1066<ul>
1067<li>
1068<p><a href="DTD/xhtml1-strict.dtd" type="text/plain">
1069XHTML-1.0-Strict</a></p>
1070</li>
1071
1072<li>
1073<p><a href="DTD/xhtml1-transitional.dtd" type="text/plain">
1074XHTML-1.0-Transitional</a></p>
1075</li>
1076
1077<li>
1078<p><a href="DTD/xhtml1-frameset.dtd" type="text/plain">
1079XHTML-1.0-Frameset</a></p>
1080</li>
1081</ul>
1082
1083<h2><a name="h-A2" id="h-A2">A.2 Entity Sets</a></h2>
1084
1085<p>The XHTML entity sets are the same as for HTML 4.0, but have
1086been modified to be valid XML 1.0 entity declarations. Note the
1087entity for the Euro currency sign (<code>&amp;euro;</code> or
1088<code>&amp;#8364;</code> or <code>&amp;#x20AC;</code>) is defined
1089as part of the special characters.</p>
1090
1091<ul>
1092<li>
1093<p><a href="DTD/xhtml-lat1.ent">Latin-1 characters</a></p>
1094</li>
1095
1096<li>
1097<p><a href="DTD/xhtml-special.ent">Special characters</a></p>
1098</li>
1099
1100<li>
1101<p><a href="DTD/xhtml-symbol.ent">Symbols</a></p>
1102</li>
1103</ul>
1104
1105<!--OddPage-->
1106<h1><a name="prohibitions" id="prohibitions">Appendix B. Element
1107Prohibitions</a></h1>
1108
1109<p><b>This appendix is normative.</b></p>
1110
1111<p>The following elements have prohibitions on which elements
1112they can contain (see <a href="#h-4.9">Section 4.9</a>). This
1113prohibition applies to all depths of nesting, i.e. it contains
1114all the descendant elements.</p>
1115
1116<dl><dt><code class="tag">a</code></dt>
1117<dd>
1118cannot contain other <code>a</code> elements.</dd>
1119<dt><code class="tag">pre</code></dt>
1120<dd>cannot contain the <code>img</code>, <code>object</code>,
1121<code>big</code>, <code>small</code>, <code>sub</code>, or <code>
1122sup</code> elements.</dd>
1123
1124<dt><code class="tag">button</code></dt>
1125<dd>cannot contain the <code>input</code>, <code>select</code>,
1126<code>textarea</code>, <code>label</code>, <code>button</code>,
1127<code>form</code>, <code>fieldset</code>, <code>iframe</code> or
1128<code>isindex</code> elements.</dd>
1129<dt><code class="tag">label</code></dt>
1130<dd>cannot contain other <code class="tag">label</code> elements.</dd>
1131<dt><code class="tag">form</code></dt>
1132<dd>cannot contain other <code>form</code> elements.</dd>
1133</dl>
1134
1135<!--OddPage-->
1136<h1><a name="guidelines" id="guidelines">Appendix C.
1137HTML Compatibility Guidelines</a></h1>
1138
1139<p><b>This appendix is informative.</b></p>
1140
1141<p>This appendix summarizes design guidelines for authors who
1142wish their XHTML documents to render on existing HTML user
1143agents.</p>
1144
1145<h2>C.1 Processing Instructions</h2>
1146<p>Be aware that processing instructions are rendered on some
1147user agents. However, also note that when the XML declaration is not included
1148in a document, the document can only use the default character encodings UTF-8
1149or UTF-16.</p>
1150
1151<h2>C.2 Empty Elements</h2>
1152<p>Include a space before the trailing <code>/</code> and <code>
1153&gt;</code> of empty elements, e.g. <code class="greenmono">
1154&lt;br&#xA0;/&gt;</code>, <code class="greenmono">
1155&lt;hr&#xA0;/&gt;</code> and <code class="greenmono">&lt;img
1156src="karen.jpg" alt="Karen"&#xA0;/&gt;</code>. Also, use the
1157minimized tag syntax for empty elements, e.g. <code class="greenmono">&lt;br /&gt;</code>, as the alternative syntax <code class="greenmono">&lt;br&gt;&lt;/br&gt;</code> allowed by XML
1158gives uncertain results in many existing user agents.</p>
1159
1160<h2>C.3 Element Minimization and Empty Element Content</h2>
1161<p>Given an empty instance of an element whose content model is
1162not <code>EMPTY</code> (for example, an empty title or paragraph)
1163do not use the minimized form (e.g. use <code class="greenmono">
1164&lt;p&gt; &lt;/p&gt;</code> and not <code class="greenmono">
1165&lt;p&#xA0;/&gt;</code>).</p>
1166
1167<h2>C.4 Embedded Style Sheets and Scripts</h2>
1168<p>Use external style sheets if your style sheet uses <code>
1169&lt;</code> or <code>&amp;</code> or <code>]]&gt;</code> or <code>--</code>. Use
1170external scripts if your script uses <code>&lt;</code> or <code>
1171&amp;</code> or <code>]]&gt;</code> or <code>--</code>. Note that XML parsers
1172are permitted to silently remove the contents of comments. Therefore, the historical
1173practice of "hiding" scripts and style sheets within comments to make the
1174documents backward compatible is likely to not work as expected in XML-based
1175implementations.</p>
1176
1177<h2>C.5 Line Breaks within Attribute Values</h2>
1178<p>Avoid line breaks and multiple whitespace characters within
1179attribute values. These are handled inconsistently by user
1180agents.</p>
1181
1182<h2>C.6 Isindex</h2>
1183<p>Don't include more than one <code>isindex</code> element in
1184the document <code>head</code>. The <code>isindex</code> element
1185is deprecated in favor of the <code>input</code> element.</p>
1186
1187<h2>C.7 The <code>lang</code> and <code>xml:lang</code> Attributes</h2>
1188<p>Use both the <code>lang</code> and <code>xml:lang</code>
1189attributes when specifying the language of an element. The value
1190of the <code>xml:lang</code> attribute takes precedence.</p>
1191
1192<h2>C.8 Fragment Identifiers</h2>
1193<p>In XML, <abbr title="Uniform Resource Identifiers">URIs</abbr> [<a href="#ref-rfc2396">RFC2396</a>] that end with fragment identifiers of the form
1194<code>"#foo"</code> do not refer to elements with an attribute
1195<code>name="foo"</code>; rather, they refer to elements with an
1196attribute defined to be of type <code>ID</code>, e.g., the <code>
1197id</code> attribute in HTML 4.0. Many existing HTML clients don't
1198support the use of <code>ID</code>-type attributes in this way,
1199so identical values may be supplied for both of these attributes to ensure
1200maximum forward and backward compatibility (e.g., <code class="greenmono">&lt;a id="foo" name="foo"&gt;...&lt;/a&gt;</code>).</p>
1201
1202<p>Further, since the set of
1203legal values for attributes of type <code>ID</code> is much smaller than
1204for those of type <code>CDATA</code>, the type of the <code>name</code>
1205attribute has been changed to <code>NMTOKEN</code>. This attribute is
1206constrained such that it can only have the same values as type
1207<code>ID</code>, or as the <code>Name</code> production in XML 1.0 Section
12082.5, production 5. Unfortunately, this constraint cannot be expressed in the
1209XHTML 1.0 DTDs. Because of this change, care must be taken when
1210converting existing HTML documents. The values of these attributes
1211must be unique within the document, valid, and any references to these
1212fragment identifiers (both
1213internal and external) must be updated should the values be changed during
1214conversion.</p>
1215<p>Finally, note that XHTML 1.0 has deprecated the
1216<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>
1217elements, and it will be
1218removed from XHTML in subsequent versions.</p>
1219
1220<h2>C.9 Character Encoding</h2>
1221<p>To specify a character encoding in the document, use both the
1222encoding attribute specification on the xml declaration (e.g.
1223<code class="greenmono">&lt;?xml version="1.0"
1224encoding="EUC-JP"?&gt;</code>) and a meta http-equiv statement
1225(e.g. <code class="greenmono">&lt;meta http-equiv="Content-type"
1226content='text/html; charset="EUC-JP"'&#xA0;/&gt;</code>). The
1227value of the encoding attribute of the xml processing instruction
1228takes precedence.</p>
1229
1230<h2>C.10 Boolean Attributes</h2>
1231<p>Some HTML user agents are unable to interpret boolean
1232attributes when these appear in their full (non-minimized) form,
1233as required by XML 1.0. Note this problem doesn't effect user
1234agents compliant with HTML 4.0. The following attributes are
1235involved: <code>compact</code>, <code>nowrap</code>, <code>
1236ismap</code>, <code>declare</code>, <code>noshade</code>, <code>
1237checked</code>, <code>disabled</code>, <code>readonly</code>,
1238<code>multiple</code>, <code>selected</code>, <code>
1239noresize</code>, <code>defer</code>.</p>
1240
1241<h2>C.11 Document Object Model and XHTML</h2>
1242<p>
1243The Document Object Model level 1 Recommendation [<a href="#ref-dom">DOM</a>]
1244defines document object model interfaces for XML and HTML 4.0. The HTML 4.0
1245document object model specifies that HTML element and attribute names are
1246returned in upper-case. The XML document object model specifies that
1247element and attribute names are returned in the case they are specified. In
1248XHTML 1.0, elements and attributes are specified in lower-case. This apparent difference can be
1249addressed in two ways:
1250</p>
1251<ol>
1252<li>Applications that access XHTML documents served as Internet media type
1253<code>text/html</code>
1254via the <abbr title="Document Object Model">DOM</abbr> can use the HTML DOM,
1255and can rely upon element and attribute names being returned in
1256upper-case from those interfaces.</li>
1257<li>Applications that access XHTML documents served as Internet media types
1258<code>text/xml</code> or <code>application/xml</code>
1259can also use the XML DOM. Elements and attributes will be returned in lower-case.
1260Also, some XHTML elements may or may
1261not appear
1262in the object tree because they are optional in the content model
1263(e.g. the <code>tbody</code> element within
1264<code>table</code>). This occurs because in HTML 4.0 some elements were
1265permitted to be minimized such that their start and end tags are both omitted
1266(an SGML feature).
1267This is not possible in XML. Rather than require document authors to insert
1268extraneous elements, XHTML has made the elements optional.
1269Applications need to adapt to this
1270accordingly.</li>
1271</ol>
1272
1273<h2>C.12 Using Ampersands in Attribute Values</h2>
1274<p>
1275When an attribute value contains an ampersand, it must be expressed as a character
1276entity reference
1277(e.g. "<code>&amp;amp;</code>"). For example, when the
1278<code>href</code> attribute
1279of the <code>a</code> element refers to a
1280CGI script that takes parameters, it must be expressed as
1281<code>http://my.site.dom/cgi-bin/myscript.pl?class=guest&amp;amp;name=user</code>
1282rather than as
1283<code>http://my.site.dom/cgi-bin/myscript.pl?class=guest&amp;name=user</code>.
1284</p>
1285
1286<h2>C.13 Cascading Style Sheets (CSS) and XHTML</h2>
1287
1288<p>The Cascading Style Sheets level 2 Recommendation [<a href="#ref-css2">CSS2</a>] defines style
1289properties which are applied to the parse tree of the HTML or XML
1290document. Differences in parsing will produce different visual or
1291aural results, depending on the selectors used. The following hints
1292will reduce this effect for documents which are served without
1293modification as both media types:</p>
1294
1295<ol>
1296<li>
1297CSS style sheets for XHTML should use lower case element and
1298attribute names.</li>
1299
1300
1301<li>In tables, the tbody element will be inferred by the parser of an
1302HTML user agent, but not by the parser of an XML user agent. Therefore
1303you should always explicitly add a tbody element if it is referred to
1304in a CSS selector.</li>
1305
1306<li>Within the XHTML name space, user agents are expected to
1307recognize the "id" attribute as an attribute of type ID.
1308Therefore, style sheets should be able to continue using the
1309shorthand "#" selector syntax even if the user agent does not read
1310the DTD.</li>
1311
1312<li>Within the XHTML name space, user agents are expected to
1313recognize the "class" attribute. Therefore, style sheets should be
1314able to continue using the shorthand "." selector syntax.</li>
1315
1316<li>
1317CSS defines different conformance rules for HTML and XML documents;
1318be aware that the HTML rules apply to XHTML documents delivered as
1319HTML and the XML rules apply to XHTML documents delivered as XML.</li>
1320</ol>
1321<!--OddPage-->
1322<h1><a name="acks" id="acks">Appendix D.
1323Acknowledgements</a></h1>
1324
1325<p><b>This appendix is informative.</b></p>
1326
1327<p>This specification was written with the participation of the
1328members of the W3C HTML working group:</p>
1329
1330<dl>
1331<dd>Steven Pemberton, CWI (HTML Working Group Chair)<br />
1332Murray Altheim, Sun Microsystems<br />
1333Daniel Austin, CNET: The Computer Network<br />
1334Frank Boumphrey, HTML Writers Guild<br />
1335John Burger, Mitre<br />
1336Andrew W. Donoho, IBM<br />
1337Sam Dooley, IBM<br />
1338Klaus Hofrichter, GMD<br />
1339Philipp Hoschka, W3C<br />
1340Masayasu Ishikawa, W3C<br />
1341Warner ten Kate, Philips Electronics<br />
1342Peter King, Phone.com<br />
1343Paula Klante, JetForm<br />
1344Shin'ichi Matsui, W3C/Panasonic<br />
1345Shane McCarron, Applied Testing and Technology (The Open Group through August
13461999)<br />
1347Ann Navarro, HTML Writers Guild<br />
1348Zach Nies, Quark<br />
1349Dave Raggett, W3C/HP (W3C lead for HTML)<br />
1350Patrick Schmitz, Microsoft<br />
1351Sebastian Schnitzenbaumer, Stack Overflow<br />
1352Chris Wilson, Microsoft<br />
1353Ted Wugofski, Gateway 2000<br />
1354Dan Zigmond, WebTV Networks</dd>
1355</dl>
1356
1357<!--OddPage-->
1358<h1><a name="refs" id="refs">Appendix E. References</a></h1>
1359
1360<p><b>This appendix is informative.</b></p>
1361
1362<dl>
1363
1364<dt><a name="ref-css2" id="ref-css2"><b>[CSS2]</b></a></dt>
1365
1366<dd><a href="http://www.w3.org/TR/REC-CSS2">"Cascading Style Sheets, level 2 (CSS2) Specification"</a>, B.
1367Bos, H. W. Lie, C. Lilley, I. Jacobs, 12 May 1998.<br />
1368Available at: <a href="http://www.w3.org/TR/REC-CSS2">
1369http://www.w3.org/TR/REC-CSS2</a></dd>
1370
1371<dt><a name="ref-dom" id="ref-dom"><b>[DOM]</b></a></dt>
1372
1373<dd><a href="http://www.w3.org/TR/REC-DOM-Level-1">"Document Object Model (DOM) Level 1 Specification"</a>, Lauren
1374Wood <i>et al.</i>, 1 October 1998.<br />
1375Available at: <a href="http://www.w3.org/TR/REC-DOM-Level-1">
1376http://www.w3.org/TR/REC-DOM-Level-1</a></dd>
1377
1378<dt><a name="ref-html4" id="ref-html4"><b>[HTML]</b></a></dt>
1379
1380<dd><a href="http://www.w3.org/TR/1999/PR-html40-19990824">"HTML 4.01 Specification"</a>, D. Raggett, A. Le&#xA0;Hors, I.
1381Jacobs, 24 August 1999.<br />
1382Available at: <a href="http://www.w3.org/TR/1999/PR-html40-19990824">
1383http://www.w3.org/TR/1999/PR-html40-19990824</a></dd>
1384
1385<dt><a name="ref-posix" id="ref-posix"><b>[POSIX.1]</b></a></dt>
1386
1387<dd>"ISO/IEC 9945-1:1990 Information Technology - Portable
1388Operating System Interface (POSIX) - Part 1: System Application
1389Program Interface (API) [C Language]", Institute of Electrical
1390and Electronics Engineers, Inc, 1990.</dd>
1391
1392<dt><a name="ref-rfc2046" id="ref-rfc2046"><b>
1393[RFC2046]</b></a></dt>
1394
1395<dd><a href="http://www.ietf.org/rfc/rfc2046.txt">"RFC2046: Multipurpose Internet Mail Extensions (MIME) Part
1396Two: Media Types"</a>, N. Freed and N. Borenstein, November
13971996.<br />
1398Available at <a href="http://www.ietf.org/rfc/rfc2046.txt">
1399http://www.ietf.org/rfc/rfc2046.txt</a>. Note that this RFC
1400obsoletes RFC1521, RFC1522, and RFC1590.</dd>
1401
1402<dt><a name="ref-rfc2119" id="ref-rfc2119"><b>
1403[RFC2119]</b></a></dt>
1404
1405<dd><a href="http://www.ietf.org/rfc/rfc2119.txt">"RFC2119: Key words for use in RFCs to Indicate Requirement
1406Levels"</a>, S. Bradner, March 1997.<br />
1407Available at: <a href="http://www.ietf.org/rfc/rfc2119.txt">
1408http://www.ietf.org/rfc/rfc2119.txt</a></dd>
1409
1410<dt><a name="ref-rfc2376" id="ref-rfc2376"><b>
1411[RFC2376]</b></a></dt>
1412
1413<dd><a href="http://www.ietf.org/rfc/rfc2376.txt">"RFC2376: XML Media Types"</a>, E. Whitehead, M. Murata, July
14141998.<br />
1415Available at: <a href="http://www.ietf.org/rfc/rfc2376.txt">
1416http://www.ietf.org/rfc/rfc2376.txt</a></dd>
1417
1418<dt><a name="ref-rfc2396" id="ref-rfc2396"><b>
1419[RFC2396]</b></a></dt>
1420
1421<dd><a href="http://www.ietf.org/rfc/rfc2396.txt">"RFC2396: Uniform Resource Identifiers (URI): Generic
1422Syntax"</a>, T. Berners-Lee, R. Fielding, L. Masinter, August
14231998.<br />
1424This document updates RFC1738 and RFC1808.<br />
1425Available at: <a href="http://www.ietf.org/rfc/rfc2396.txt">
1426http://www.ietf.org/rfc/rfc2396.txt</a></dd>
1427
1428<dt><a name="ref-xml" id="ref-xml"><b>[XML]</b></a></dt>
1429
1430<dd><a href="http://www.w3.org/TR/REC-xml">"Extensible Markup Language (XML) 1.0 Specification"</a>, T.
1431Bray, J. Paoli, C. M. Sperberg-McQueen, 10 February 1998.<br />
1432Available at: <a href="http://www.w3.org/TR/REC-xml">
1433http://www.w3.org/TR/REC-xml</a></dd>
1434
1435<dt><a name="ref-xmlns" id="ref-xmlns"><b>[XMLNAMES]</b></a></dt>
1436
1437<dd><a href="http://www.w3.org/TR/REC-xml-names">"Namespaces in XML"</a>, T. Bray, D. Hollander, A. Layman, 14
1438January 1999.<br />
1439XML namespaces provide a simple method for qualifying names used
1440in XML documents by associating them with namespaces identified
1441by URI.<br />
1442Available at: <a href="http://www.w3.org/TR/REC-xml-names">
1443http://www.w3.org/TR/REC-xml-names</a></dd>
1444
1445</dl>
1446<p><a href="http://www.w3.org/WAI/WCAG1AAA-Conformance" title="Explanation of Level Triple-A Conformance">
1447<img height="32" width="88" src="wcag1AAA.gif" alt="Level Triple-A conformance icon, W3C-WAI Web Content Accessibility Guidelines 1.0" /></a></p>
1448<div class="navbar">
1449 <hr />
1450 <a href="#toc">table of contents</a>
1451</div>
1452</body>
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