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Daniel Veillard	686d6b6	2000-01-03 11:08:02 +0000	[diff] [blame]	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
				6	Language</title>
				7	<link rel="stylesheet"
				8	href="W3C-PR.css" type="text/css" />
				9	<style type="text/css">
				10	span.term { font-style: italic; color: rgb(0, 0, 192) }
				11	code {
				12	color: green;
				13	font-family: monospace;
				14	font-weight: bold;
				15	}
				16
				17	code.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
				38	img {
				39	color: white;
				40	border: none;
				41	}
				42
				43	div.navbar { text-align: center; }
				44	div.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; }
				51	table.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"
				60	src="w3c_home.gif" alt="W3C" /></a></p>
				61
				62	<h1 class="head"><a name="title" id="title">XHTML</a><sup>™</sup> 1.0:
				63	The 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">
				74	http://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">
				84	http://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">
				90	http://www.w3.org/TR/1999/WD-xhtml1-19991124</a></dd>
				91	<dd><a href=
				92	"http://www.w3.org/TR/1999/PR-xhtml1-19990824">
				93	http://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">
				102	Copyright</a> © 1999 <a href="http://www.w3.org/">W3C</a><sup>®</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
				106	title="World Wide Web Consortium">W3C</abbr> <a
				107	href=
				108	"http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">
				109	liability</a>, <a href=
				110	"http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">
				111	trademark</a>, <a href=
				112	"http://www.w3.org/Consortium/Legal/copyright-documents">document
				113	use</a> and <a href=
				114	"http://www.w3.org/Consortium/Legal/copyright-software">software
				115	licensing</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
				122	Language">XHTML</abbr> 1.0, a reformulation of HTML
				123	4.0 as an XML 1.0 application, and three <abbr title="Document Type
				124	Definition">DTDs</abbr> corresponding to
				125	the ones defined by HTML 4.0. The semantics of the elements and
				126	their attributes are defined in the W3C Recommendation for HTML
				127	4.0. These semantics provide the foundation for future
				128	extensibility of XHTML. Compatibility with existing HTML user
				129	agents 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
				134	of its publication. Other documents may supersede this document. The
				135	latest 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
				138	a revision of the Proposed Recommendation dated <a
				139	href= "http://www.w3.org/TR/1999/PR-xhtml1-19990824/">24 August
				140	1999</a> incorporating changes as a result of comments from the Proposed
				141	Recommendation review, and
				142	comments and further deliberations of the W3C HTML Working Group. A
				143	<a href="xhtml1-diff-19991210.html">diff-marked version</a> from the previous
				144	proposed 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">
				148	Proposed Recommendation</a> review period. From that date until 8 January
				149	2000,
				150	W3C Advisory Committee representatives are encouraged
				151	to review this specification and return comments in their completed
				152	ballots to w3c-html-review@w3.org. Please send any comments of a
				153	confidential nature in separate email to w3t-html@w3.org, which is
				154	visible to the Team only.</p>
				155
				156	<p>No sooner than 14 days after the end of the review period, the
				157	Director will announce the document's disposition: it may become a W3C
				158	Recommendation (possibly with minor changes), it may revert to Working
				159	Draft status, or it may be dropped as a W3C work item.</p>
				160
				161	<p>Publication as a Proposed Recommendation does not imply endorsement
				162	by the W3C membership. This is still a draft document and may be
				163	updated, replaced or obsoleted by other documents at any time. It is
				164	inappropriate to cite W3C Proposed Recommendation as other than "work
				165	in 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
				169	the <a href="http://www.w3.org/MarkUp/Group/">HTML Working
				170	Group</a> <i>(<a href="http://cgi.w3.org/MemberAccess/">members
				171	only</a>)</i> are discussed in the <a href=
				172	"http://www.w3.org/MarkUp/Group/HTMLcharter">HTML Working Group
				173	charter</a> <i>(<a href="http://cgi.w3.org/MemberAccess/">members
				174	only</a>)</i>.</p>
				175
				176	<p>A list of current W3C Recommendations and other technical documents
				177	can be found at <a
				178	href="http://www.w3.org/TR">http://www.w3.org/TR</a>.</p>
				179
				180	<p>Public discussion on <abbr title="HyperText Markup
				181	Language">HTML</abbr> features takes place on the mailing list <a
				182	href="mailto:www-html@w3.org"> www-html@w3.org</a> (<a href=
				183	"http://lists.w3.org/Archives/Public/www-html/">archive</a>). The W3C
				184	staff contact for work on HTML is <a href= "mailto:dsr@w3.org">Dave
				185	Raggett</a>.</p>
				186
				187	<p>Please report errors in this document to <a
				188	href="mailto:www-html-editor@w3.org">www-html-editor@w3.org</a>.</p>
				189
				190	<p>The list of known errors in this specification is available at <a
				191	href="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
				252	Prohibitions</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
				266	reproduce, 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,
				267	and ultimately are designed to work in conjunction with XML-based user agents.
				268	The details of this family and its evolution are
				269	discussed in more detail in the section on <a href="#future">Future
				270	Directions</a>. </p>
				271
				272	<p>XHTML 1.0 (this specification) is the first document type in the XHTML
				273	family. It is a reformulation of the three HTML 4.0 document types as
				274	applications of XML 1.0 <a href="#ref-xml"> [XML]</a>. It is intended
				275	to be used as a language for content that is both XML-conforming and, if some
				276	simple <a href="#guidelines">guidelines</a> are followed,
				277	operates in HTML 4.0 conforming user agents. Developers who migrate
				278	their 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,
				282	edited, and validated with standard XML tools.</li>
				283	<li>XHTML documents can be written to
				284	to operate as well or better than they did before in existing
				285	HTML 4.0-conforming user agents as well as in new, XHTML 1.0 conforming user
				286	agents.</li>
				287	<li>XHTML documents can utilize applications (e.g. scripts and applets) that rely
				288	upon either the HTML Document Object Model or the XML Document Object Model <a
				289	href="#ref-dom">[DOM]</a>.</li>
				290	<li>As the XHTML family evolves, documents conforming to XHTML 1.0 will be more
				291	likely 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
				295	migrating to XHTML today, content developers can enter the XML world with all
				296	of its attendant benefits, while still remaining confident in their
				297	content'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
				302	Generalized Markup Language">SGML</abbr> (Standard
				303	Generalized Markup Language) application conforming to
				304	International Standard <abbr title="Organization for International
				305	Standardization">ISO</abbr> 8879, and is widely regarded as the
				306	standard publishing language of the World Wide Web.</p>
				307
				308	<p>SGML is a language for describing markup languages,
				309	particularly those used in electronic document exchange, document
				310	management, and document publishing. HTML is an example of a
				311	language defined in SGML.</p>
				312
				313	<p>SGML has been around since the middle 1980's and has remained
				314	quite stable. Much of this stability stems from the fact that the
				315	language is both feature-rich and flexible. This flexibility,
				316	however, comes at a price, and that price is a level of
				317	complexity that has inhibited its adoption in a diversity of
				318	environments, including the World Wide Web.</p>
				319
				320	<p>HTML, as originally conceived, was to be a language for the
				321	exchange of scientific and other technical documents, suitable
				322	for use by non-document specialists. HTML addressed the problem
				323	of SGML complexity by specifying a small set of structural and
				324	semantic tags suitable for authoring relatively simple documents.
				325	In addition to simplifying the document structure, HTML added
				326	support for hypertext. Multimedia capabilities were added
				327	later.</p>
				328
				329	<p>In a remarkably short space of time, HTML became wildly
				330	popular and rapidly outgrew its original purpose. Since HTML's
				331	inception, there has been rapid invention of new elements for use
				332	within HTML (as a standard) and for adapting HTML to vertical,
				333	highly specialized, markets. This plethora of new elements has
				334	led to compatibility problems for documents across different
				335	platforms.</p>
				336
				337	<p>As the heterogeneity of both software and platforms rapidly
				338	proliferate, it is clear that the suitability of 'classic' HTML
				339	4.0 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>™</sup> is the shorthand for Extensible Markup
				344	Language, and is an acronym of Extensible Markup Language <a
				345	href="#ref-xml">[XML]</a>.</p>
				346
				347	<p>XML was conceived as a means of regaining the power and
				348	flexibility of SGML without most of its complexity. Although a
				349	restricted form of SGML, XML nonetheless preserves most of SGML's
				350	power and richness, and yet still retains all of SGML's commonly
				351	used features.</p>
				352
				353	<p>While retaining these beneficial features, XML removes many of
				354	the more complex features of SGML that make the authoring and
				355	design 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
				360	benefits of migrating to XHTML in general are:</p>
				361
				362	<ul>
				363	<li>Document developers and user agent designers are constantly
				364	discovering new ways to express their ideas through new markup. In XML, it is
				365	relatively easy to introduce new elements or additional element
				366	attributes. The XHTML family is designed to accommodate these extensions
				367	through XHTML modules and techniques for developing new XHTML-conforming
				368	modules (described in the forthcoming XHTML Modularization specification).
				369	These modules will permit the combination of existing and
				370	new feature sets when developing content and when designing new user
				371	agents.</li>
				372
				373	<li>Alternate ways of accessing the Internet are constantly being
				374	introduced. Some estimates indicate that by the year 2002, 75% of
				375	Internet document viewing will be carried out on these alternate
				376	platforms. The XHTML family is designed with general user agent
				377	interoperability in mind. Through a new user agent and document profiling
				378	mechanism, servers, proxies, and user agents will be able to perform
				379	best effort content transformation. Ultimately, it will be possible to
				380	develop XHTML-conforming content that is usable by any XHTML-conforming
				381	user 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
				390	terms extend the definitions in <a href="#ref-rfc2119">
				391	[RFC2119]</a> in ways based upon similar definitions in ISO/<abbr
				392	title="International Electro-technical Commission">IEC</abbr>
				393	9945-1:1990 <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
				399	to the implementation to define [and document] the corresponding
				400	requirements for correct document construction.</dd>
				401
				402	<dt>May</dt>
				403
				404	<dd>With respect to implementations, the word "may" is to be
				405	interpreted as an optional feature that is not required in this
				406	specification but can be provided. With respect to <a href=
				407	"#docconf">Document Conformance</a>, the word "may" means that
				408	the optional feature must not be used. The term "optional" has
				409	the same definition as "may".</dd>
				410
				411	<dt>Must</dt>
				412
				413	<dd>In this specification, the word "must" is to be interpreted
				414	as a mandatory requirement on the implementation or on Strictly
				415	Conforming 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
				421	be used by Conforming Documents nor to be supported by a
				422	Conforming User Agents.</dd>
				423
				424	<dt>Should</dt>
				425
				426	<dd>With respect to implementations, the word "should" is to be
				427	interpreted as an implementation recommendation, but not a
				428	requirement. With respect to documents, the word "should" is to
				429	be interpreted as recommended programming practice for documents
				430	and 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
				435	facility is supported, it behaves as specified by this
				436	specification.</dd>
				437
				438	<dt>Unspecified</dt>
				439
				440	<dd>When a value or behavior is unspecified, the specification
				441	defines no portability requirements for a facility on an
				442	implementation even when faced with a document that uses the
				443	facility. A document that requires specific behavior in such an
				444	instance, rather than tolerating any behavior when using that
				445	facility, 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
				454	DTD. An attribute's type and value range, including a possible
				455	default 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
				460	declarations that, as a collection, defines the legal structure,
				461	<span class="term">elements</span>, and <span class="term">
				462	attributes</span> that are available for use in a document that
				463	complies to the DTD.</dd>
				464
				465	<dt>Document</dt>
				466
				467	<dd>A document is a stream of data that, after being combined
				468	with any other streams it references, is structured such that it
				469	holds information contained within <span class="term">
				470	elements</span> that are organized as defined in the associated
				471	<span class="term">DTD</span>. See <a href="#docconf">Document
				472	Conformance</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
				478	defined in the <span class="term">DTD</span>, and additional
				479	semantics may be defined in the prose description of the
				480	element.</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
				486	associated with those <span class="term">elements</span> and
				487	<span class="term">attributes</span>. An implementation
				488	supporting that functionality is said to provide the necessary
				489	facilities.</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
				495	this specification. See <a href="#uaconf">User Agent
				496	Conformance</a> for more information.</dd>
				497
				498	<dt>Parsing</dt>
				499
				500	<dd>Parsing is the act whereby a <span class="term">
				501	document</span> is scanned, and the information contained within
				502	the <span class="term">document</span> is filtered into the
				503	context of the <span class="term">elements</span> in which the
				504	information is structured.</dd>
				505
				506	<dt>Rendering</dt>
				507
				508	<dd>Rendering is the act whereby the information in a <span
				509	class="term">document</span> is presented. This presentation is
				510	done in the form most appropriate to the environment (e.g.
				511	aurally, visually, in print).</dd>
				512
				513	<dt>User Agent</dt>
				514
				515	<dd>A user agent is an <span class="term">implementation</span>
				516	that 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">
				522	documents</span> are verified against the associated <span class=
				523	"term">DTD</span>, ensuring that the structure, use of <span
				524	class="term">elements</span>, and use of <span class="term">
				525	attributes</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
				531	is 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
				533	the XML 1.0 Recommendation <a href="#ref-xml">[XML]</a>.
				534	Basically, this definition states that elements, delimited by
				535	their start and end tags, are nested properly within one
				536	another.</dd>
				537	</dl>
				538
				539	<!--OddPage-->
				540	<h1><a name="normative" id="normative">3. Normative Definition of
				541	XHTML 1.0</a></h1>
				542
				543	<h2><a name="docconf" id="docconf">3.1 Document
				544	Conformance</a></h2>
				545
				546	<p>This version of XHTML provides a definition of strictly
				547	conforming XHTML documents, which are restricted to tags and
				548	attributes from the XHTML namespace. See <a href=
				549	"#well-formed">Section 3.1.2</a> for information on using XHTML
				550	with other namespaces, for instance, to include metadata
				551	expressed 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
				554	Documents</a></h3>
				555
				556	<p>A Strictly Conforming XHTML Document is a document that
				557	requires only the facilities described as mandatory in this
				558	specification. Such a document must meet all of the following
				559	criteria:</p>
				560
				561	<ol>
				562	<li>
				563	<p>It must validate against one of the three DTDs found in <a
				564	href="#dtds">Appendix A</a>.</p>
				565	</li>
				566
				567	<li>
				568	<p>The root element of the document must be <code>
				569	<html></code>.</p>
				570	</li>
				571
				572	<li>
				573	<p>The root element of the document must designate the XHTML
				574	namespace using the <code>xmlns</code> attribute <a href=
				575	"#ref-xmlns">[XMLNAMES]</a>. The namespace for XHTML is
				576	defined 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
				582	the root element. The public identifier included in
				583	the DOCTYPE declaration must reference one of the three DTDs
				584	found in <a href="#dtds">Appendix A</a> using the respective
				585	Formal Public Identifier. The system identifier may be changed to reflect
				586	local system conventions.</p>
				587
				588	<pre>
				589	<!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>
				592
				593	<!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>
				596
				597	<!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>
				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	<?xml version="1.0" encoding="UTF-8"?>
				609	<!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">
				612	<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
				613	<head>
				614	<title>Virtual Library</title>
				615	</head>
				616	<body>
				617	<p>Moved to <a href="http://vlib.org/">vlib.org</a>.</p>
				618	</body>
				619	</html></pre>
				620	</div>
				621
				622	<p>Note that in this example, the XML declaration is included. An XML
				623	declaration like the one above is
				624	not required in all XML documents. XHTML document authors are strongly encouraged to use XML declarations in all their documents. Such a declaration is required
				625	when the character encoding of the document is other than the default UTF-8 or
				626	UTF-16.</p>
				627
				628	<h3><a name="well-formed" id="well-formed">3.1.2 Using XHTML with
				629	other namespaces</a></h3>
				630
				631	<p>The XHTML namespace may be used with other XML namespaces
				632	as per <a href="#ref-xmlns">[XMLNAMES]</a>, although such
				633	documents are not strictly conforming XHTML 1.0 documents as
				634	defined above. Future work by W3C will address ways to specify
				635	conformance for documents involving multiple namespaces.</p>
				636
				637	<p>The following example shows the way in which XHTML 1.0 could
				638	be used in conjunction with the MathML Recommendation:</p>
				639
				640	<div class="good">
				641	<pre>
				642	<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
				643	<head>
				644	<title>A Math Example</title>
				645	</head>
				646	<body>
				647	<p>The following is MathML markup:</p>
				648	<math xmlns="http://www.w3.org/1998/Math/MathML">
				649	<apply> <log/>
				650	<logbase>
				651	<cn> 3 </cn>
				652	</logbase>
				653	<ci> x </ci>
				654	</apply>
				655	</math>
				656	</body>
				657	</html>
				658	</pre>
				659	</div>
				660
				661	<p>The following example shows the way in which XHTML 1.0 markup
				662	could be incorporated into another XML namespace:</p>
				663
				664	<div class="good">
				665	<pre>
				666	<?xml version="1.0" encoding="UTF-8"?>
				667	<!-- initially, the default namespace is "books" -->
				668	<book xmlns='urn:loc.gov:books'
				669	xmlns:isbn='urn:ISBN:0-395-36341-6' xml:lang="en" lang="en">
				670	<title>Cheaper by the Dozen</title>
				671	<isbn:number>1568491379</isbn:number>
				672	<notes>
				673	<!-- make HTML the default namespace for a hypertext commentary -->
				674	<p xmlns='http://www.w3.org/1999/xhtml'>
				675	This is also available <a href="http://www.w3.org/">online</a>.
				676	</p>
				677	</notes>
				678	</book>
				679	</pre>
				680	</div>
				681
				682	<h2><a name="uaconf" id="uaconf">3.2 User Agent
				683	Conformance</a></h2>
				684
				685	<p>A conforming user agent must meet all of the following
				686	criteria:</p>
				687
				688	<ol>
				689	<li>In order to be consistent with the XML 1.0 Recommendation <a
				690	href="#ref-xml">[XML]</a>, the user agent must parse and evaluate
				691	an XHTML document for well-formedness. If the user agent claims
				692	to be a validating user agent, it must also validate documents
				693	against 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">
				697	facilities</a> defined within this specification or required by
				698	this specification through normative reference, it must do so in
				699	ways consistent with the facilities' definition.</li>
				700
				701	<li>When a user agent processes an XHTML document as generic XML,
				702	it shall only recognize attributes of type
				703	<code>ID</code> (e.g. the <code>id</code> attribute on most XHTML elements)
				704	as fragment identifiers.</li>
				705
				706	<li>If a user agent encounters an element it does not recognize,
				707	it must render the element's content.</li>
				708
				709	<li>If a user agent encounters an attribute it does not
				710	recognize, 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
				714	recognize, it must use the default attribute value.</li>
				715
				716	<li>If it encounters an entity reference (other than one
				717	of the predefined entities) for which the User Agent has
				718	processed no declaration (which could happen if the declaration
				719	is in the external subset which the User Agent hasn't read), the entity
				720	reference should be rendered as the characters (starting
				721	with the ampersand and ending with the semi-colon) that
				722	make up the entity reference.</li>
				723
				724	<li>When rendering content, User Agents that encounter
				725	characters 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>
				728	The following characters are defined in [XML] as whitespace characters:
				729
				730	<ul>
				731	<li>Space (&#x0020;)</li>
				732	<li>Tab (&#x0009;)</li>
				733	<li>Carriage return (&#x000D;)</li>
				734	<li>Line feed (&#x000A;)</li>
				735	</ul>
				736
				737	<p>
				738	The XML processor normalizes different system's line end codes into one
				739	single line-feed character, that is passed up to the application. The XHTML
				740	user agent in addition, must treat the following characters as whitespace:
				741	</p>
				742
				743	<ul>
				744	<li>Form feed (&#x000C;)</li>
				745	<li>Zero-width space (&#x200B;)</li>
				746	</ul>
				747
				748	<p>
				749	In elements where the 'xml:space' attribute is set to 'preserve', the user
				750	agent must leave all whitespace characters intact (with the exception of
				751	leading and trailing whitespace characters, which should be removed).
				752	Otherwise, whitespace
				753	is handled according to the following rules:
				754	</p>
				755
				756	<ul>
				757	<li>
				758	All whitespace surrounding block elements should be removed.
				759	</li>
				760	<li>
				761	Comments are removed entirely and do not affect whitespace handling. One
				762	whitespace character on either side of a comment is treated as two white
				763	space characters.
				764	</li>
				765	<li>
				766	Leading 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
				769	space (except when the 'xml:space' attribute is set to 'preserve').
				770	</li>
				771	<li>
				772	A sequence of white space characters must be reduced to a single space
				773	character (except when the 'xml:space' attribute is set to 'preserve').
				774	</li>
				775	<li>
				776	With regard to rendition,
				777	the User Agent should render the content in a
				778	manner appropriate to the language in which the content is written.
				779	In languages whose primary script is Latinate, the ASCII space
				780	character is typically used to encode both grammatical word boundaries and
				781	typographic whitespace; in languages whose script is related to Nagari
				782	(e.g., Sanskrit, Thai, etc.), grammatical boundaries may be encoded using
				783	the ZW 'space' character, but will not typically be represented by
				784	typographic whitespace in rendered output; languages using Arabiform scripts
				785	may encode typographic whitespace using a space character, but may also use
				786	the ZW space character to delimit 'internal' grammatical boundaries (what
				787	look like words in Arabic to an English eye frequently encode several words,
				788	e.g. 'kitAbuhum' = 'kitAbu-hum' = 'book them' == their book); and languages
				789	in the Chinese script tradition typically neither encode such delimiters nor
				790	use typographic whitespace in this way.
				791	</li>
				792	</ul>
				793
				794	<p>Whitespace in attribute values is processed according to <a
				795	href="#ref-xml">[XML]</a>.</p>
				796	</li>
				797	</ol>
				798
				799	<!--OddPage-->
				800	<h1><a name="diffs" id="diffs">4. Differences with HTML
				801	4.0</a></h1>
				802
				803	<p>Due to the fact that XHTML is an XML application, certain
				804	practices that were perfectly legal in SGML-based HTML 4.0 <a
				805	href="#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
				808	well-formed</a></h2>
				809
				810	<p><a href="#wellformed">Well-formedness</a> is a new concept
				811	introduced by <a href="#ref-xml">[XML]</a>. Essentially this
				812	means that all elements must either have closing tags or be
				813	written in a special form (as described below), and that all the
				814	elements must nest.</p>
				815
				816	<p>Although overlapping is illegal in SGML, it was widely
				817	tolerated in existing browsers.</p>
				818
				819	<div class="good">
				820	<p><strong><em>CORRECT: nested elements.</em></strong></p>
				821
				822	<p><p>here is an emphasized
				823	<em>paragraph</em>.</p></p>
				824	</div>
				825
				826	<div class="bad">
				827	<p><strong><em>INCORRECT: overlapping elements</em></strong></p>
				828
				829	<p><p>here is an emphasized
				830	<em>paragraph.</p></em></p>
				831	</div>
				832
				833	<h2><a name="h-4.2" id="h-4.2">4.2 Element and attribute
				834	names must be in lower case</a></h2>
				835
				836	<p>XHTML documents must use lower case for all HTML element and
				837	attribute names. This difference is necessary because XML is
				838	case-sensitive e.g. <li> and <LI> are different
				839	tags.</p>
				840
				841	<h2><a name="h-4.3" id="h-4.3">4.3 For non-empty elements,
				842	end tags are required</a></h2>
				843
				844	<p>In SGML-based HTML 4.0 certain elements were permitted to omit
				845	the end tag; with the elements that followed implying closure.
				846	This omission is not permitted in XML-based XHTML. All elements
				847	other than those declared in the DTD as <code>EMPTY</code> must
				848	have an end tag.</p>
				849
				850	<div class="good">
				851	<p><strong><em>CORRECT: terminated elements</em></strong></p>
				852
				853	<p><p>here is a paragraph.</p><p>here is
				854	another paragraph.</p></p>
				855	</div>
				856
				857	<div class="bad">
				858	<p><strong><em>INCORRECT: unterminated elements</em></strong></p>
				859
				860	<p><p>here is a paragraph.<p>here is another
				861	paragraph.</p>
				862	</div>
				863
				864	<h2><a name="h-4.4" id="h-4.4">4.4 Attribute values must
				865	always be quoted</a></h2>
				866
				867	<p>All attribute values must be quoted, even those which appear
				868	to be numeric.</p>
				869
				870	<div class="good">
				871	<p><strong><em>CORRECT: quoted attribute values</em></strong></p>
				872
				873	<p><table rows="3"></p>
				874	</div>
				875
				876	<div class="bad">
				877	<p><strong><em>INCORRECT: unquoted attribute values</em></strong></p>
				878
				879	<p><table rows=3></p>
				880	</div>
				881
				882	<h2><a name="h-4.5" id="h-4.5">4.5 Attribute
				883	Minimization</a></h2>
				884
				885	<p>XML does not support attribute minimization. Attribute-value
				886	pairs must be written in full. Attribute names such as <code>
				887	compact</code> and <code>checked</code> cannot occur in elements
				888	without their value being specified.</p>
				889
				890	<div class="good">
				891	<p><strong><em>CORRECT: unminimized attributes</em></strong></p>
				892
				893	<p><dl compact="compact"></p>
				894	</div>
				895
				896	<div class="bad">
				897	<p><strong><em>INCORRECT: minimized attributes</em></strong></p>
				898
				899	<p><dl compact></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>/></code>. For instance,
				905	<code><br/></code> or <code><hr></hr></code>. See <a
				906	href="#guidelines">HTML Compatibility Guidelines</a> for information on ways to
				907	ensure 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><br/><hr/></p>
				913	</div>
				914
				915	<div class="bad">
				916	<p><strong><em>INCORRECT: unterminated empty tags</em></strong></p>
				917
				918	<p><br><hr></p>
				919	</div>
				920
				921	<h2><a name="h-4.7" id="h-4.7">4.7 Whitespace handling in
				922	attribute values</a></h2>
				923
				924	<p>In attribute values, user agents will strip leading and
				925	trailing whitespace from attribute values and map sequences
				926	of one or more whitespace characters (including line breaks) to
				927	a single inter-word space (an ASCII space character for western
				928	scripts). See <a href="http://www.w3.org/TR/REC-xml#AVNormalize">
				929	Section 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
				932	elements</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><</code> and
				936	<code>&</code> will be treated as the start of markup, and
				937	entities such as <code>&lt;</code> and <code>&amp;</code>
				938	will be recognized as entity references by the XML processor to
				939	<code><</code> and <code>&</code> respectively. Wrapping
				940	the content of the script or style element within a <code>
				941	CDATA</code> marked section avoids the expansion of these
				942	entities.</p>
				943
				944	<div class="good">
				945	<pre>
				946	<script>
				947	<![CDATA[
				948	... unescaped script content ...
				949	]]>
				950	</script>
				951	</pre>
				952	</div>
				953
				954	<p><code>CDATA</code> sections are recognized by the XML
				955	processor 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">
				958	Section 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
				962	documents.</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
				967	elements from being contained within an element. Such
				968	prohibitions (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
				972	to any descendant depth. It is not possible to spell out such
				973	prohibitions in XML. Even though these prohibitions cannot be
				974	defined in the DTD, certain elements should not be nested. A
				975	summary of such elements and the elements that should not be
				976	nested in them is found in the normative <a href="#prohibitions">
				977	Appendix B</a>.</p>
				978
				979	<h2><a name="h-4.10" id="h-4.10">4.10 The elements with 'id' and 'name'
				980	attributes</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>.
				986	HTML 4.0 also introduced
				987	the <code>id</code> attribute. Both of these attributes are designed to be
				988	used as fragment identifiers.</p>
				989	<p>In XML, fragment identifiers are of type <code>ID</code>, and
				990	there can only be a single attribute of type <code>ID</code> per element.
				991	Therefore, in XHTML 1.0 the <code>id</code>
				992	attribute is defined to be of type <code>ID</code>. In order to
				993	ensure that XHTML 1.0 documents are well-structured XML documents, XHTML 1.0
				994	documents MUST use the <code>id</code> attribute when defining fragment
				995	identifiers, even on elements that historically have also had a
				996	<code>name</code> attribute.
				997	See the <a href="#guidelines">HTML Compatibility
				998	Guidelines</a> for information on ensuring such anchors are backwards
				999	compatible 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
				1002	elements is formally deprecated, and will be removed in a
				1003	subsequent 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
				1009	compatible with existing user agents, in practice this is easy to
				1010	accomplish. Guidelines for creating compatible documents can be
				1011	found in <a href="#guidelines">Appendix 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
				1015	recommended MIME labeling for XML-based applications
				1016	has yet to be resolved.</p>
				1017
				1018	<p>However, XHTML Documents which follow the guidelines set forth
				1019	in <a href="#guidelines">Appendix C</a>, "HTML Compatibility Guidelines" may be
				1020	labeled with the Internet Media Type "text/html", as they
				1021	are compatible with most HTML browsers. This document
				1022	makes no recommendation about MIME labeling of other
				1023	XHTML 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
				1029	that will extend and subset XHTML, in order to support a wide
				1030	range of new devices and applications, by defining modules and
				1031	specifying a mechanism for combining these modules. This
				1032	mechanism will enable the extension and sub-setting of XHTML 1.0
				1033	in 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
				1038	agents to other platforms, it is clear that not all of the XHTML
				1039	elements will be required on all platforms. For example a hand
				1040	held device or a cell-phone may only support a subset of XHTML
				1041	elements.</p>
				1042
				1043	<p>The process of modularization breaks XHTML up into a series of
				1044	smaller element sets. These elements can then be recombined to
				1045	meet 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
				1050	Extensibility</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
				1073	Profiles</a></h2>
				1074
				1075	<p>A document profile specifies the syntax and semantics of a set
				1076	of documents. Conformance to a document profile provides a basis
				1077	for interoperability guarantees. The document profile specifies
				1078	the facilities required to process documents of that type, e.g.
				1079	which image formats can be used, levels of scripting, style sheet
				1080	support, and so on.</p>
				1081
				1082	<p>For product designers this enables various groups to define
				1083	their own standard profile.</p>
				1084
				1085	<p>For authors this will obviate the need to write several
				1086	different versions of documents for different clients.</p>
				1087
				1088	<p>For special groups such as chemists, medical doctors, or
				1089	mathematicians this allows a special profile to be built using
				1090	standard HTML elements plus a group of elements geared to the
				1091	specialist'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
				1100	specification. The complete set of DTD files together with an XML
				1101	declaration 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
				1105	Definitions</a></h2>
				1106
				1107	<p>These DTDs approximate the HTML 4.0 DTDs. It is likely that
				1108	when the DTDs are modularized, a method of DTD construction will
				1109	be 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">
				1114	XHTML-1.0-Strict</a></p>
				1115	</li>
				1116
				1117	<li>
				1118	<p><a href="DTD/xhtml1-transitional.dtd" type="text/plain">
				1119	XHTML-1.0-Transitional</a></p>
				1120	</li>
				1121
				1122	<li>
				1123	<p><a href="DTD/xhtml1-frameset.dtd" type="text/plain">
				1124	XHTML-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
				1131	been modified to be valid XML 1.0 entity declarations. Note the
				1132	entity for the Euro currency sign (<code>&euro;</code> or
				1133	<code>&#8364;</code> or <code>&#x20AC;</code>) is defined
				1134	as 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
				1152	Prohibitions</a></h1>
				1153
				1154	<p><b>This appendix is normative.</b></p>
				1155
				1156	<p>The following elements have prohibitions on which elements
				1157	they can contain (see <a href="#h-4.9">Section 4.9</a>). This
				1158	prohibition applies to all depths of nesting, i.e. it contains
				1159	all the descendant elements.</p>
				1160
				1161	<dl><dt><code class="tag">a</code></dt>
				1162	<dd>
				1163	cannot 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>
				1167	sup</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.
				1182	HTML 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
				1187	wish their XHTML documents to render on existing HTML user
				1188	agents.</p>
				1189
				1190	<h2>C.1 Processing Instructions</h2>
				1191	<p>Be aware that processing instructions are rendered on some
				1192	user agents. However, also note that when the XML declaration is not included
				1193	in a document, the document can only use the default character encodings UTF-8
				1194	or UTF-16.</p>
				1195
				1196	<h2>C.2 Empty Elements</h2>
				1197	<p>Include a space before the trailing <code>/</code> and <code>
				1198	></code> of empty elements, e.g. <code class="greenmono">
				1199	<br /></code>, <code class="greenmono">
				1200	<hr /></code> and <code class="greenmono"><img
				1201	src="karen.jpg" alt="Karen" /></code>. Also, use the
				1202	minimized tag syntax for empty elements, e.g. <code class=
				1203	"greenmono"><br /></code>, as the alternative syntax <code
				1204	class="greenmono"><br></br></code> allowed by XML
				1205	gives 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
				1209	not <code>EMPTY</code> (for example, an empty title or paragraph)
				1210	do not use the minimized form (e.g. use <code class="greenmono">
				1211	<p> </p></code> and not <code class="greenmono">
				1212	<p /></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	<</code> or <code>&</code> or <code>]]></code> or <code>--</code>. Use
				1217	external scripts if your script uses <code><</code> or <code>
				1218	&</code> or <code>]]></code> or <code>--</code>. Note that XML parsers
				1219	are permitted to silently remove the contents of comments. Therefore, the historical
				1220	practice of "hiding" scripts and style sheets within comments to make the
				1221	documents backward compatible is likely to not work as expected in XML-based
				1222	implementations.</p>
				1223
				1224	<h2>C.5 Line Breaks within Attribute Values</h2>
				1225	<p>Avoid line breaks and multiple whitespace characters within
				1226	attribute values. These are handled inconsistently by user
				1227	agents.</p>
				1228
				1229	<h2>C.6 Isindex</h2>
				1230	<p>Don't include more than one <code>isindex</code> element in
				1231	the document <code>head</code>. The <code>isindex</code> element
				1232	is 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>
				1236	attributes when specifying the language of an element. The value
				1237	of 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
				1243	attribute defined to be of type <code>ID</code>, e.g., the <code>
				1244	id</code> attribute in HTML 4.0. Many existing HTML clients don't
				1245	support the use of <code>ID</code>-type attributes in this way,
				1246	so identical values may be supplied for both of these attributes to ensure
				1247	maximum forward and backward compatibility (e.g., <code class=
				1248	"greenmono"><a id="foo" name="foo">...</a></code>).</p>
				1249
				1250	<p>Further, since the set of
				1251	legal values for attributes of type <code>ID</code> is much smaller than
				1252	for those of type <code>CDATA</code>, the type of the <code>name</code>
				1253	attribute has been changed to <code>NMTOKEN</code>. This attribute is
				1254	constrained 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
				1256	2.5, production 5. Unfortunately, this constraint cannot be expressed in the
				1257	XHTML 1.0 DTDs. Because of this change, care must be taken when
				1258	converting existing HTML documents. The values of these attributes
				1259	must be unique within the document, valid, and any references to these
				1260	fragment identifiers (both
				1261	internal and external) must be updated should the values be changed during
				1262	conversion.</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>
				1265	elements, and it will be
				1266	removed 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
				1270	encoding attribute specification on the xml declaration (e.g.
				1271	<code class="greenmono"><?xml version="1.0"
				1272	encoding="EUC-JP"?></code>) and a meta http-equiv statement
				1273	(e.g. <code class="greenmono"><meta http-equiv="Content-type"
				1274	content='text/html; charset="EUC-JP"' /></code>). The
				1275	value of the encoding attribute of the xml processing instruction
				1276	takes precedence.</p>
				1277
				1278	<h2>C.10 Boolean Attributes</h2>
				1279	<p>Some HTML user agents are unable to interpret boolean
				1280	attributes when these appear in their full (non-minimized) form,
				1281	as required by XML 1.0. Note this problem doesn't effect user
				1282	agents compliant with HTML 4.0. The following attributes are
				1283	involved: <code>compact</code>, <code>nowrap</code>, <code>
				1284	ismap</code>, <code>declare</code>, <code>noshade</code>, <code>
				1285	checked</code>, <code>disabled</code>, <code>readonly</code>,
				1286	<code>multiple</code>, <code>selected</code>, <code>
				1287	noresize</code>, <code>defer</code>.</p>
				1288
				1289	<h2>C.11 Document Object Model and XHTML</h2>
				1290	<p>
				1291	The Document Object Model level 1 Recommendation [<a href="#ref-dom">DOM</a>]
				1292	defines document object model interfaces for XML and HTML 4.0. The HTML 4.0
				1293	document object model specifies that HTML element and attribute names are
				1294	returned in upper-case. The XML document object model specifies that
				1295	element and attribute names are returned in the case they are specified. In
				1296	XHTML 1.0, elements and attributes are specified in lower-case. This apparent difference can be
				1297	addressed in two ways:
				1298	</p>
				1299	<ol>
				1300	<li>Applications that access XHTML documents served as Internet media type
				1301	<code>text/html</code>
				1302	via the <abbr title="Document Object Model">DOM</abbr> can use the HTML DOM,
				1303	and can rely upon element and attribute names being returned in
				1304	upper-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>
				1307	can also use the XML DOM. Elements and attributes will be returned in lower-case.
				1308	Also, some XHTML elements may or may
				1309	not appear
				1310	in 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
				1313	permitted to be minimized such that their start and end tags are both omitted
				1314	(an SGML feature).
				1315	This is not possible in XML. Rather than require document authors to insert
				1316	extraneous elements, XHTML has made the elements optional.
				1317	Applications need to adapt to this
				1318	accordingly.</li>
				1319	</ol>
				1320
				1321	<h2>C.12 Using Ampersands in Attribute Values</h2>
				1322	<p>
				1323	When an attribute value contains an ampersand, it must be expressed as a character
				1324	entity reference
				1325	(e.g. "<code>&amp;</code>"). For example, when the
				1326	<code>href</code> attribute
				1327	of the <code>a</code> element refers to a
				1328	CGI script that takes parameters, it must be expressed as
				1329	<code>http://my.site.dom/cgi-bin/myscript.pl?class=guest&amp;name=user</code>
				1330	rather than as
				1331	<code>http://my.site.dom/cgi-bin/myscript.pl?class=guest&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
				1337	properties which are applied to the parse tree of the HTML or XML
				1338	document. Differences in parsing will produce different visual or
				1339	aural results, depending on the selectors used. The following hints
				1340	will reduce this effect for documents which are served without
				1341	modification as both media types:</p>
				1342
				1343	<ol>
				1344	<li>
				1345	CSS style sheets for XHTML should use lower case element and
				1346	attribute names.</li>
				1347
				1348
				1349	<li>In tables, the tbody element will be inferred by the parser of an
				1350	HTML user agent, but not by the parser of an XML user agent. Therefore
				1351	you should always explicitely add a tbody element if it is referred to
				1352	in a CSS selector.</li>
				1353
				1354	<li>Within the XHTML name space, user agents are expected to
				1355	recognize the "id" attribute as an attribute of type ID.
				1356	Therefore, style sheets should be able to continue using the
				1357	shorthand "#" selector syntax even if the user agent does not read
				1358	the DTD.</li>
				1359
				1360	<li>Within the XHTML name space, user agents are expected to
				1361	recognize the "class" attribute. Therefore, style sheets should be
				1362	able to continue using the shorthand "." selector syntax.</li>
				1363
				1364	<li>
				1365	CSS defines different conformance rules for HTML and XML documents;
				1366	be aware that the HTML rules apply to XHTML documents delivered as
				1367	HTML 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.
				1371	Acknowledgements</a></h1>
				1372
				1373	<p><b>This appendix is informative.</b></p>
				1374
				1375	<p>This specification was written with the participation of the
				1376	members of the W3C HTML working group:</p>
				1377
				1378	<dl>
				1379	<dd>Steven Pemberton, CWI (HTML Working Group Chair)<br />
				1380	Murray Altheim, Sun Microsystems<br />
				1381	Daniel Austin, CNET: The Computer Network<br />
				1382	Frank Boumphrey, HTML Writers Guild<br />
				1383	John Burger, Mitre<br />
				1384	Andrew W. Donoho, IBM<br />
				1385	Sam Dooley, IBM<br />
				1386	Klaus Hofrichter, GMD<br />
				1387	Philipp Hoschka, W3C<br />
				1388	Masayasu Ishikawa, W3C<br />
				1389	Warner ten Kate, Philips Electronics<br />
				1390	Peter King, Phone.com<br />
				1391	Paula Klante, JetForm<br />
				1392	Shin'ichi Matsui, W3C/Panasonic<br />
				1393	Shane McCarron, Applied Testing and Technology (The Open Group through August
				1394	1999)<br />
				1395	Ann Navarro, HTML Writers Guild<br />
				1396	Zach Nies, Quark<br />
				1397	Dave Raggett, W3C/HP (W3C lead for HTML)<br />
				1398	Patrick Schmitz, Microsoft<br />
				1399	Sebastian Schnitzenbaumer, Stack Overflow<br />
				1400	Chris Wilson, Microsoft<br />
				1401	Ted Wugofski, Gateway 2000<br />
				1402	Dan 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.
				1415	Bos, H. W. Lie, C. Lilley, I. Jacobs, 12 May 1998.<br />
				1416	Available at: <a href="http://www.w3.org/TR/REC-CSS2">
				1417	http://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
				1422	Wood <i>et al.</i>, 1 October 1998.<br />
				1423	Available at: <a href="http://www.w3.org/TR/REC-DOM-Level-1">
				1424	http://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 Hors, I.
				1429	Jacobs, 24 August 1999.<br />
				1430	Available at: <a href="http://www.w3.org/TR/1999/PR-html40-19990824">
				1431	http://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
				1436	Operating System Interface (POSIX) - Part 1: System Application
				1437	Program Interface (API) [C Language]", Institute of Electrical
				1438	and 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
				1444	Two: Media Types"</a>, N. Freed and N. Borenstein, November
				1445	1996.<br />
				1446	Available at <a href="http://www.ietf.org/rfc/rfc2046.txt">
				1447	http://www.ietf.org/rfc/rfc2046.txt</a>. Note that this RFC
				1448	obsoletes 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
				1454	Levels"</a>, S. Bradner, March 1997.<br />
				1455	Available at: <a href="http://www.ietf.org/rfc/rfc2119.txt">
				1456	http://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
				1462	1998.<br />
				1463	Available at: <a href="http://www.ietf.org/rfc/rfc2376.txt">
				1464	http://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
				1470	Syntax"</a>, T. Berners-Lee, R. Fielding, L. Masinter, August
				1471	1998.<br />
				1472	This document updates RFC1738 and RFC1808.<br />
				1473	Available at: <a href="http://www.ietf.org/rfc/rfc2396.txt">
				1474	http://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.
				1479	Bray, J. Paoli, C. M. Sperberg-McQueen, 10 February 1998.<br />
				1480	Available at: <a href="http://www.w3.org/TR/REC-xml">
				1481	http://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
				1486	January 1999.<br />
				1487	XML namespaces provide a simple method for qualifying names used
				1488	in XML documents by associating them with namespaces identified
				1489	by URI.<br />
				1490	Available at: <a href="http://www.w3.org/TR/REC-xml-names">
				1491	http://www.w3.org/TR/REC-xml-names</a></dd>
				1492
				1493	</dl>
				1494	<p><a href="http://www.w3.org/WAI/WCAG1AAA-Conformance"
				1495	title="Explanation of Level Triple-A Conformance">
				1496	<img height="32" width="88"
				1497	src="wcag1AAA.gif"
				1498	alt="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>
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				1505