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>™</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> © 1999 <a href="http://www.w3.org/">W3C</a><sup>®</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 w3c-html-review@w3.org. Please send any comments of a 137confidential nature in separate email to w3t-html@w3.org, 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:www-html@w3.org"> www-html@w3.org</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:dsr@w3.org">Dave 163Raggett</a>.</p> 164 165<p>Please report errors in this document to <a href="mailto:www-html-editor@w3.org">www-html-editor@w3.org</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>™</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 A</a>.</p> 528</li> 529 530<li> 531<p>The root element of the document must be <code> 532<html></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 A</a> using the respective 547Formal Public Identifier. The system identifier may be changed to reflect 548local system conventions.</p> 549 550<pre> 551<!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> 554 555<!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> 558 559<!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> 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<?xml version="1.0" encoding="UTF-8"?> 571<!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"> 574<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 575 <head> 576 <title>Virtual Library</title> 577 </head> 578 <body> 579 <p>Moved to <a href="http://vlib.org/">vlib.org</a>.</p> 580 </body> 581</html></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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 605 <head> 606 <title>A Math Example</title> 607 </head> 608 <body> 609 <p>The following is MathML markup:</p> 610 <math xmlns="http://www.w3.org/1998/Math/MathML"> 611 <apply> <log/> 612 <logbase> 613 <cn> 3 </cn> 614 </logbase> 615 <ci> x </ci> 616 </apply> 617 </math> 618 </body> 619</html> 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<?xml version="1.0" encoding="UTF-8"?> 629<!-- initially, the default namespace is "books" --> 630<book xmlns='urn:loc.gov:books' 631 xmlns:isbn='urn:ISBN:0-395-36341-6' xml:lang="en" lang="en"> 632 <title>Cheaper by the Dozen</title> 633 <isbn:number>1568491379</isbn:number> 634 <notes> 635 <!-- make HTML the default namespace for a hypertext commentary --> 636 <p xmlns='http://www.w3.org/1999/xhtml'> 637 This is also available <a href="http://www.w3.org/">online</a>. 638 </p> 639 </notes> 640</book> 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 (&#x0020;)</li> 693<li>Tab (&#x0009;)</li> 694<li>Carriage return (&#x000D;)</li> 695<li>Line feed (&#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 (&#x000C;)</li> 706<li>Zero-width space (&#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><p>here is an emphasized 782<em>paragraph</em>.</p></p> 783</div> 784 785<div class="bad"> 786<p><strong><em>INCORRECT: overlapping elements</em></strong></p> 787 788<p><p>here is an emphasized 789<em>paragraph.</p></em></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. <li> and <LI> 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><p>here is a paragraph.</p><p>here is 813another paragraph.</p></p> 814</div> 815 816<div class="bad"> 817<p><strong><em>INCORRECT: unterminated elements</em></strong></p> 818 819<p><p>here is a paragraph.<p>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><table rows="3"></p> 833</div> 834 835<div class="bad"> 836<p><strong><em>INCORRECT: unquoted attribute values</em></strong></p> 837 838<p><table rows=3></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><dl compact="compact"></p> 853</div> 854 855<div class="bad"> 856<p><strong><em>INCORRECT: minimized attributes</em></strong></p> 857 858<p><dl compact></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>/></code>. For instance, 864<code><br/></code> or <code><hr></hr></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><br/><hr/></p> 871</div> 872 873<div class="bad"> 874<p><strong><em>INCORRECT: unterminated empty tags</em></strong></p> 875 876<p><br><hr></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><</code> and 894<code>&</code> will be treated as the start of markup, and 895entities such as <code>&lt;</code> and <code>&amp;</code> 896will be recognized as entity references by the XML processor to 897<code><</code> and <code>&</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<script> 905 <![CDATA[ 906 ... unescaped script content ... 907 ]]> 908 </script> 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 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 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>&euro;</code> or 1088<code>&#8364;</code> or <code>&#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></code> of empty elements, e.g. <code class="greenmono"> 1154<br /></code>, <code class="greenmono"> 1155<hr /></code> and <code class="greenmono"><img 1156src="karen.jpg" alt="Karen" /></code>. Also, use the 1157minimized tag syntax for empty elements, e.g. <code class="greenmono"><br /></code>, as the alternative syntax <code class="greenmono"><br></br></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<p> </p></code> and not <code class="greenmono"> 1165<p /></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<</code> or <code>&</code> or <code>]]></code> or <code>--</code>. Use 1170external scripts if your script uses <code><</code> or <code> 1171&</code> or <code>]]></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"><a id="foo" name="foo">...</a></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"><?xml version="1.0" 1224encoding="EUC-JP"?></code>) and a meta http-equiv statement 1225(e.g. <code class="greenmono"><meta http-equiv="Content-type" 1226content='text/html; charset="EUC-JP"' /></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;</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;name=user</code> 1282rather than as 1283<code>http://my.site.dom/cgi-bin/myscript.pl?class=guest&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 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> 1453</html> 1454
