release/src/router/libxml2/result/valid/xhtml1.xhtml

   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
   7 Language</title>
   8 <link rel="stylesheet" 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 }
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  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" src="w3c_home.gif" alt="W3C" /></a></p>
  60
  61 <h1 class="head"><a name="title" id="title">XHTML</a><sup>&#x2122;</sup> 1.0:
  62 The 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">
  72 http://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">
  82 http://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">
  87 http://www.w3.org/TR/1999/WD-xhtml1-19991124</a></dd>
  88 <dd><a href="http://www.w3.org/TR/1999/PR-xhtml1-19990824">
  89 http://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">
  97 Copyright</a> &copy; 1999 <a href="http://www.w3.org/">W3C</a><sup>&reg;</sup>
  98 (<a href="http://www.lcs.mit.edu/">MIT</a>, <a href="http://www.inria.fr/">INRIA</a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. <abbr title="World Wide Web Consortium">W3C</abbr> <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">
  99 liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">
 100 trademark</a>, <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document
 101 use</a> and <a href="http://www.w3.org/Consortium/Legal/copyright-software">software
 102 licensing</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
 109 4.0 as an XML 1.0 application, and three <abbr title="Document Type Definition">DTDs</abbr> corresponding to
 110 the ones defined by HTML 4.0. The semantics of the elements and
 111 their attributes are defined in the W3C Recommendation for HTML
 112 4.0. These semantics provide the foundation for future
 113 extensibility of XHTML. Compatibility with existing HTML user
 114 agents 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
 119 of its publication. Other documents may supersede this document. The
 120 latest 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
 123 a revision of the Proposed Recommendation dated <a href="http://www.w3.org/TR/1999/PR-xhtml1-19990824/">24 August
 124 1999</a> incorporating changes as a result of comments from the Proposed
 125 Recommendation review, and
 126 comments and further deliberations of the W3C HTML Working Group. A
 127 <a href="xhtml1-diff-19991210.html">diff-marked version</a> from the previous
 128 proposed 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">
 132 Proposed Recommendation</a> review period. From that date until 8 January
 133 2000,
 134 W3C Advisory Committee representatives are encouraged
 135 to review this specification and return comments in their completed
 136 ballots to w3c-html-review@w3.org. Please send any comments of a
 137 confidential nature in separate email to w3t-html@w3.org, which is
 138 visible to the Team only.</p>
 139
 140 <p>No sooner than 14 days after the end of the review period, the
 141 Director will announce the document's disposition: it may become a W3C
 142 Recommendation (possibly with minor changes), it may revert to Working
 143 Draft status, or it may be dropped as a W3C work item.</p>
 144
 145 <p>Publication as a Proposed Recommendation does not imply endorsement
 146 by the W3C membership.  This is still a draft document and may be
 147 updated, replaced or obsoleted by other documents at any time. It is
 148 inappropriate to cite W3C Proposed Recommendation as other than "work
 149 in 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
 152 the <a href="http://www.w3.org/MarkUp/Group/">HTML Working
 153 Group</a> <i>(<a href="http://cgi.w3.org/MemberAccess/">members
 154 only</a>)</i> are discussed in the <a href="http://www.w3.org/MarkUp/Group/HTMLcharter">HTML Working Group
 155 charter</a> <i>(<a href="http://cgi.w3.org/MemberAccess/">members
 156 only</a>)</i>.</p>
 157
 158 <p>A list of current W3C Recommendations and other technical documents
 159 can 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
 162 staff contact for work on HTML is <a href="mailto:dsr@w3.org">Dave
 163 Raggett</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
 228 Prohibitions</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
 242 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,
 243 and ultimately are designed to work in conjunction with XML-based user agents.
 244 The details of this family and its evolution are
 245 discussed in more detail in the section on <a href="#future">Future
 246 Directions</a>. </p>
 247
 248 <p>XHTML 1.0 (this specification) is the first document type in the XHTML
 249 family. It is a reformulation of the three HTML 4.0 document types as
 250 applications of XML 1.0 <a href="#ref-xml"> [XML]</a>. It is intended
 251 to be used as a language for content that is both XML-conforming and, if some
 252 simple <a href="#guidelines">guidelines</a> are followed,
 253 operates in HTML 4.0 conforming user agents. Developers who migrate
 254 their 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,
 258 edited, and validated with standard XML tools.</li>
 259 <li>XHTML documents can be written to
 260 to operate as well or better than they did before in existing
 261 HTML 4.0-conforming user agents as well as in new, XHTML 1.0 conforming user
 262 agents.</li>
 263 <li>XHTML documents can utilize applications (e.g. scripts and applets) that rely
 264 upon 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
 266 likely 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
 270 migrating to XHTML today, content developers can enter the XML world with all
 271 of its attendant benefits, while still remaining confident in their
 272 content'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
 277 Generalized Markup Language) application conforming to
 278 International Standard <abbr title="Organization for International Standardization">ISO</abbr> 8879, and is widely regarded as the
 279 standard publishing language of the World Wide Web.</p>
 280
 281 <p>SGML is a language for describing markup languages,
 282 particularly those used in electronic document exchange, document
 283 management, and document publishing. HTML is an example of a
 284 language defined in SGML.</p>
 285
 286 <p>SGML has been around since the middle 1980's and has remained
 287 quite stable. Much of this stability stems from the fact that the
 288 language is both feature-rich and flexible. This flexibility,
 289 however, comes at a price, and that price is a level of
 290 complexity that has inhibited its adoption in a diversity of
 291 environments, including the World Wide Web.</p>
 292
 293 <p>HTML, as originally conceived, was to be a language for the
 294 exchange of scientific and other technical documents, suitable
 295 for use by non-document specialists. HTML addressed the problem
 296 of SGML complexity by specifying a small set of structural and
 297 semantic tags suitable for authoring relatively simple documents.
 298 In addition to simplifying the document structure, HTML added
 299 support for hypertext. Multimedia capabilities were added
 300 later.</p>
 301
 302 <p>In a remarkably short space of time, HTML became wildly
 303 popular and rapidly outgrew its original purpose. Since HTML's
 304 inception, there has been rapid invention of new elements for use
 305 within HTML (as a standard) and for adapting HTML to vertical,
 306 highly specialized, markets. This plethora of new elements has
 307 led to compatibility problems for documents across different
 308 platforms.</p>
 309
 310 <p>As the heterogeneity of both software and platforms rapidly
 311 proliferate, it is clear that the suitability of 'classic' HTML
 312 4.0 for use on these platforms is somewhat limited.</p>
 313
 314 <h2><a name="xml" id="xml">1.2 What is XML?</a></h2>
 315
 316 <p>XML<sup>&#x2122;</sup> is the shorthand for Extensible Markup
 317 Language, 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
 320 flexibility of SGML without most of its complexity. Although a
 321 restricted form of SGML, XML nonetheless preserves most of SGML's
 322 power and richness, and yet still retains all of SGML's commonly
 323 used features.</p>
 324
 325 <p>While retaining these beneficial features, XML removes many of
 326 the more complex features of SGML that make the authoring and
 327 design 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
 332 benefits of migrating to XHTML in general are:</p>
 333
 334 <ul>
 335 <li>Document developers and user agent designers are constantly
 336 discovering new ways to express their ideas through new markup. In XML, it is
 337 relatively easy to introduce new elements or additional element
 338 attributes.  The XHTML family is designed to accommodate these extensions
 339 through XHTML modules and techniques for developing new XHTML-conforming
 340 modules (described in the forthcoming XHTML Modularization specification).
 341 These modules will permit the combination of existing and
 342 new feature sets when developing content and when designing new user
 343 agents.</li>
 344
 345 <li>Alternate ways of accessing the Internet are constantly being
 346 introduced.  Some estimates indicate that by the year 2002, 75% of
 347 Internet document viewing will be carried out on these alternate
 348 platforms.  The XHTML family is designed with general user agent
 349 interoperability in mind. Through a new user agent and document profiling
 350 mechanism, servers, proxies, and user agents will be able to perform
 351 best effort content transformation. Ultimately, it will be possible to
 352 develop XHTML-conforming content that is usable by any XHTML-conforming
 353 user 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
 362 terms 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>
 364 9945-1:1990 <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
 370 to the implementation to define [and document] the corresponding
 371 requirements for correct document construction.</dd>
 372
 373 <dt>May</dt>
 374
 375 <dd>With respect to implementations, the word "may" is to be
 376 interpreted as an optional feature that is not required in this
 377 specification but can be provided. With respect to <a href="#docconf">Document Conformance</a>, the word "may" means that
 378 the optional feature must not be used. The term "optional" has
 379 the same definition as "may".</dd>
 380
 381 <dt>Must</dt>
 382
 383 <dd>In this specification, the word "must" is to be interpreted
 384 as a mandatory requirement on the implementation or on Strictly
 385 Conforming 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
 391 be used by Conforming Documents nor to be supported by a
 392 Conforming User Agents.</dd>
 393
 394 <dt>Should</dt>
 395
 396 <dd>With respect to implementations, the word "should" is to be
 397 interpreted as an implementation recommendation, but not a
 398 requirement. With respect to documents, the word "should" is to
 399 be interpreted as recommended programming practice for documents
 400 and 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
 405 facility is supported, it behaves as specified by this
 406 specification.</dd>
 407
 408 <dt>Unspecified</dt>
 409
 410 <dd>When a value or behavior is unspecified, the specification
 411 defines no portability requirements for a facility on an
 412 implementation even when faced with a document that uses the
 413 facility. A document that requires specific behavior in such an
 414 instance, rather than tolerating any behavior when using that
 415 facility, 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
 424 DTD. An attribute's type and value range, including a possible
 425 default 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
 430 declarations that, as a collection, defines the legal structure,
 431 <span class="term">elements</span>, and <span class="term">
 432 attributes</span> that are available for use in a document that
 433 complies to the DTD.</dd>
 434
 435 <dt>Document</dt>
 436
 437 <dd>A document is a stream of data that, after being combined
 438 with any other streams it references, is structured such that it
 439 holds information contained within <span class="term">
 440 elements</span> that are organized as defined in the associated
 441 <span class="term">DTD</span>. See <a href="#docconf">Document
 442 Conformance</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
 448 defined in the <span class="term">DTD</span>, and additional
 449 semantics may be defined in the prose description of the
 450 element.</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
 456 associated with those <span class="term">elements</span> and
 457 <span class="term">attributes</span>. An implementation
 458 supporting that functionality is said to provide the necessary
 459 facilities.</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
 465 this specification. See <a href="#uaconf">User Agent
 466 Conformance</a> for more information.</dd>
 467
 468 <dt>Parsing</dt>
 469
 470 <dd>Parsing is the act whereby a <span class="term">
 471 document</span> is scanned, and the information contained within
 472 the <span class="term">document</span> is filtered into the
 473 context of the <span class="term">elements</span> in which the
 474 information 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
 479 done in the form most appropriate to the environment (e.g.
 480 aurally, visually, in print).</dd>
 481
 482 <dt>User Agent</dt>
 483
 484 <dd>A user agent is an <span class="term">implementation</span>
 485 that 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">
 490 documents</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">
 491 attributes</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
 497 is structured according to the rules defined in <a href="http://www.w3.org/TR/REC-xml#sec-well-formed">Section 2.1</a> of
 498 the XML 1.0 Recommendation <a href="#ref-xml">[XML]</a>.
 499 Basically, this definition states that elements, delimited by
 500 their start and end tags, are nested properly within one
 501 another.</dd>
 502 </dl>
 503
 504 <!--OddPage-->
 505 <h1><a name="normative" id="normative">3. Normative Definition of
 506 XHTML 1.0</a></h1>
 507
 508 <h2><a name="docconf" id="docconf">3.1 Document
 509 Conformance</a></h2>
 510
 511 <p>This version of XHTML provides a definition of strictly
 512 conforming XHTML documents, which are restricted to tags and
 513 attributes from the XHTML namespace. See <a href="#well-formed">Section 3.1.2</a> for information on using XHTML
 514 with other namespaces, for instance, to include metadata
 515 expressed 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
 518 Documents</a></h3>
 519
 520 <p>A Strictly Conforming XHTML Document is a document that
 521 requires only the facilities described as mandatory in this
 522 specification. Such a document must meet all of the following
 523 criteria:</p>
 524
 525 <ol>
 526 <li>
 527 <p>It must validate against one of the three DTDs found in <a href="#dtds">Appendix&#xA0;A</a>.</p>
 528 </li>
 529
 530 <li>
 531 <p>The root element of the document must be <code>
 532 &lt;html&gt;</code>.</p>
 533 </li>
 534
 535 <li>
 536 <p>The root element of the document must designate the XHTML
 537 namespace using the <code>xmlns</code> attribute <a href="#ref-xmlns">[XMLNAMES]</a>. The namespace for XHTML is
 538 defined 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
 544 the root element. The public identifier included in
 545 the DOCTYPE declaration must reference one of the three DTDs
 546 found in <a href="#dtds">Appendix&#xA0;A</a> using the respective
 547 Formal Public Identifier. The system identifier may be changed to reflect
 548 local system conventions.</p>
 549
 550 <pre>
 551 &lt;!DOCTYPE html
 552      PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
 553      "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-strict.dtd&gt;
 554
 555 &lt;!DOCTYPE html
 556      PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
 557      "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-transitional.dtd&gt;
 558
 559 &lt;!DOCTYPE html
 560      PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
 561      "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-frameset.dtd&gt;
 562 </pre>
 563 </li>
 564 </ol>
 565
 566 <p>Here is an example of a minimal XHTML document.</p>
 567
 568 <div class="good">
 569 <pre>
 570 &lt;?xml version="1.0" encoding="UTF-8"?&gt;
 571 &lt;!DOCTYPE html
 572      PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
 573     "http://www.w3.org/TR/1999/PR-xhtml1-19991210/DTD/xhtml1-strict.dtd"&gt;
 574 &lt;html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"&gt;
 575   &lt;head&gt;
 576     &lt;title&gt;Virtual Library&lt;/title&gt;
 577   &lt;/head&gt;
 578   &lt;body&gt;
 579     &lt;p&gt;Moved to &lt;a href="http://vlib.org/"&gt;vlib.org&lt;/a&gt;.&lt;/p&gt;
 580   &lt;/body&gt;
 581 &lt;/html&gt;</pre>
 582 </div>
 583
 584 <p>Note that in this example, the XML declaration is included. An XML
 585 declaration like the one above is
 586 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
 587 when the character encoding of the document is other than the default UTF-8 or
 588 UTF-16.</p>
 589
 590 <h3><a name="well-formed" id="well-formed">3.1.2 Using XHTML with
 591 other namespaces</a></h3>
 592
 593 <p>The XHTML namespace may be used with other XML namespaces
 594 as per <a href="#ref-xmlns">[XMLNAMES]</a>, although such
 595 documents are not strictly conforming XHTML 1.0 documents as
 596 defined above. Future work by W3C will address ways to specify
 597 conformance for documents involving multiple namespaces.</p>
 598
 599 <p>The following example shows the way in which XHTML 1.0 could
 600 be used in conjunction with the MathML Recommendation:</p>
 601
 602 <div class="good">
 603 <pre>
 604 &lt;html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"&gt;
 605   &lt;head&gt;
 606     &lt;title&gt;A Math Example&lt;/title&gt;
 607   &lt;/head&gt;
 608   &lt;body&gt;
 609     &lt;p&gt;The following is MathML markup:&lt;/p&gt;
 610     &lt;math xmlns="http://www.w3.org/1998/Math/MathML"&gt;
 611       &lt;apply&gt; &lt;log/&gt;
 612         &lt;logbase&gt;
 613           &lt;cn&gt; 3 &lt;/cn&gt;
 614         &lt;/logbase&gt;
 615         &lt;ci&gt; x &lt;/ci&gt;
 616       &lt;/apply&gt;
 617     &lt;/math&gt;
 618   &lt;/body&gt;
 619 &lt;/html&gt;
 620 </pre>
 621 </div>
 622
 623 <p>The following example shows the way in which XHTML 1.0 markup
 624 could be incorporated into another XML namespace:</p>
 625
 626 <div class="good">
 627 <pre>
 628 &lt;?xml version="1.0" encoding="UTF-8"?&gt;
 629 &lt;!-- initially, the default namespace is "books" --&gt;
 630 &lt;book xmlns='urn:loc.gov:books'
 631     xmlns:isbn='urn:ISBN:0-395-36341-6' xml:lang="en" lang="en"&gt;
 632   &lt;title&gt;Cheaper by the Dozen&lt;/title&gt;
 633   &lt;isbn:number&gt;1568491379&lt;/isbn:number&gt;
 634   &lt;notes&gt;
 635     &lt;!-- make HTML the default namespace for a hypertext commentary --&gt;
 636     &lt;p xmlns='http://www.w3.org/1999/xhtml'&gt;
 637         This is also available &lt;a href="http://www.w3.org/"&gt;online&lt;/a&gt;.
 638     &lt;/p&gt;
 639   &lt;/notes&gt;
 640 &lt;/book&gt;
 641 </pre>
 642 </div>
 643
 644 <h2><a name="uaconf" id="uaconf">3.2 User Agent
 645 Conformance</a></h2>
 646
 647 <p>A conforming user agent must meet all of the following
 648 criteria:</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
 652 an XHTML document for well-formedness. If the user agent claims
 653 to be a validating user agent, it must also validate documents
 654 against 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">
 658 facilities</a> defined within this specification or required by
 659 this specification through normative reference, it must do so in
 660 ways consistent with the facilities' definition.</li>
 661
 662 <li>When a user agent processes an XHTML document as generic XML,
 663 it shall only recognize attributes of type
 664 <code>ID</code> (e.g. the <code>id</code> attribute on most XHTML elements)
 665 as fragment identifiers.</li>
 666
 667 <li>If a user agent encounters an element it does not recognize,
 668 it must render the element's content.</li>
 669
 670 <li>If a user agent encounters an attribute it does not
 671 recognize, 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
 675 recognize, it must use the default attribute value.</li>
 676
 677 <li>If it encounters an entity reference (other than one
 678 of the predefined entities) for which the User Agent has
 679 processed no declaration (which could happen if the declaration
 680 is in the external subset which the User Agent hasn't read), the entity
 681 reference should be rendered as the characters (starting
 682 with the ampersand and ending with the semi-colon) that
 683 make up the entity reference.</li>
 684
 685 <li>When rendering content, User Agents that encounter
 686 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>
 687
 688 <li>
 689 The following characters are defined in [XML] as whitespace characters:
 690
 691 <ul>
 692 <li>Space (&amp;#x0020;)</li>
 693 <li>Tab (&amp;#x0009;)</li>
 694 <li>Carriage return (&amp;#x000D;)</li>
 695 <li>Line feed (&amp;#x000A;)</li>
 696 </ul>
 697
 698 <p>
 699 The XML processor normalizes different system's line end codes into one
 700 single line-feed character, that is passed up to the application. The XHTML
 701 user agent in addition, must treat the following characters as whitespace:
 702 </p>
 703
 704 <ul>
 705 <li>Form feed (&amp;#x000C;)</li>
 706 <li>Zero-width space (&amp;#x200B;)</li>
 707 </ul>
 708
 709 <p>
 710 In elements where the 'xml:space' attribute is set to 'preserve', the user
 711 agent must leave all whitespace characters intact (with the exception of
 712 leading and trailing whitespace characters, which should be removed).
 713 Otherwise, whitespace
 714 is handled according to the following rules:
 715 </p>
 716
 717 <ul>
 718 <li>
 719 All whitespace surrounding block elements should be removed.
 720 </li>
 721 <li>
 722 Comments are removed entirely and do not affect whitespace handling. One
 723 whitespace character on either side of a comment is treated as two white
 724 space characters.
 725 </li>
 726 <li>
 727 Leading 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
 730 space (except when the 'xml:space' attribute is set to 'preserve').
 731 </li>
 732 <li>
 733 A sequence of white space characters must be reduced to a single space
 734 character (except when the 'xml:space' attribute is set to 'preserve').
 735 </li>
 736 <li>
 737 With regard to rendition,
 738 the User Agent should render the content in a
 739 manner appropriate to the language in which the content is written.
 740 In languages whose primary script is Latinate, the ASCII space
 741 character is typically used to encode both grammatical word boundaries and
 742 typographic whitespace; in languages whose script is related to Nagari
 743 (e.g., Sanskrit, Thai, etc.), grammatical boundaries may be encoded using
 744 the ZW 'space' character, but will not typically be represented by
 745 typographic whitespace in rendered output; languages using Arabiform scripts
 746 may encode typographic whitespace using a space character, but may also use
 747 the ZW space character to delimit 'internal' grammatical boundaries (what
 748 look like words in Arabic to an English eye frequently encode several words,
 749 e.g. 'kitAbuhum' = 'kitAbu-hum' = 'book them' == their book); and languages
 750 in the Chinese script tradition typically neither encode such delimiters nor
 751 use 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
 761 4.0</a></h1>
 762
 763 <p>Due to the fact that XHTML is an XML application, certain
 764 practices 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
 767 well-formed</a></h2>
 768
 769 <p><a href="#wellformed">Well-formedness</a> is a new concept
 770 introduced by <a href="#ref-xml">[XML]</a>. Essentially this
 771 means that all elements must either have closing tags or be
 772 written in a special form (as described below), and that all the
 773 elements must nest.</p>
 774
 775 <p>Although overlapping is illegal in SGML, it was widely
 776 tolerated in existing browsers.</p>
 777
 778 <div class="good">
 779 <p><strong><em>CORRECT: nested elements.</em></strong></p>
 780
 781 <p>&lt;p&gt;here is an emphasized
 782 &lt;em&gt;paragraph&lt;/em&gt;.&lt;/p&gt;</p>
 783 </div>
 784
 785 <div class="bad">
 786 <p><strong><em>INCORRECT: overlapping elements</em></strong></p>
 787
 788 <p>&lt;p&gt;here is an emphasized
 789 &lt;em&gt;paragraph.&lt;/p&gt;&lt;/em&gt;</p>
 790 </div>
 791
 792 <h2><a name="h-4.2" id="h-4.2">4.2 Element and attribute
 793 names must be in lower case</a></h2>
 794
 795 <p>XHTML documents must use lower case for all HTML element and
 796 attribute names. This difference is necessary because XML is
 797 case-sensitive e.g. &lt;li&gt; and &lt;LI&gt; are different
 798 tags.</p>
 799
 800 <h2><a name="h-4.3" id="h-4.3">4.3 For non-empty elements,
 801 end tags are required</a></h2>
 802
 803 <p>In SGML-based HTML 4.0 certain elements were permitted to omit
 804 the end tag; with the elements that followed implying closure.
 805 This omission is not permitted in XML-based XHTML. All elements
 806 other than those declared in the DTD as <code>EMPTY</code> must
 807 have an end tag.</p>
 808
 809 <div class="good">
 810 <p><strong><em>CORRECT: terminated elements</em></strong></p>
 811
 812 <p>&lt;p&gt;here is a paragraph.&lt;/p&gt;&lt;p&gt;here is
 813 another paragraph.&lt;/p&gt;</p>
 814 </div>
 815
 816 <div class="bad">
 817 <p><strong><em>INCORRECT: unterminated elements</em></strong></p>
 818
 819 <p>&lt;p&gt;here is a paragraph.&lt;p&gt;here is another
 820 paragraph.</p>
 821 </div>
 822
 823 <h2><a name="h-4.4" id="h-4.4">4.4 Attribute values must
 824 always be quoted</a></h2>
 825
 826 <p>All attribute values must be quoted, even those which appear
 827 to be numeric.</p>
 828
 829 <div class="good">
 830 <p><strong><em>CORRECT: quoted attribute values</em></strong></p>
 831
 832 <p>&lt;table rows="3"&gt;</p>
 833 </div>
 834
 835 <div class="bad">
 836 <p><strong><em>INCORRECT: unquoted attribute values</em></strong></p>
 837
 838 <p>&lt;table rows=3&gt;</p>
 839 </div>
 840
 841 <h2><a name="h-4.5" id="h-4.5">4.5 Attribute
 842 Minimization</a></h2>
 843
 844 <p>XML does not support attribute minimization. Attribute-value
 845 pairs must be written in full. Attribute names such as <code>
 846 compact</code> and <code>checked</code> cannot occur in elements
 847 without their value being specified.</p>
 848
 849 <div class="good">
 850 <p><strong><em>CORRECT: unminimized attributes</em></strong></p>
 851
 852 <p>&lt;dl compact="compact"&gt;</p>
 853 </div>
 854
 855 <div class="bad">
 856 <p><strong><em>INCORRECT: minimized attributes</em></strong></p>
 857
 858 <p>&lt;dl compact&gt;</p>
 859 </div>
 860
 861 <h2><a name="h-4.6" id="h-4.6">4.6 Empty Elements</a></h2>
 862
 863 <p>Empty elements must either have an end tag or the start tag must end with <code>/&gt;</code>. For instance,
 864 <code>&lt;br/&gt;</code> or <code>&lt;hr&gt;&lt;/hr&gt;</code>. See <a href="#guidelines">HTML Compatibility Guidelines</a> for information on ways to
 865 ensure this is backward compatible with HTML 4.0 user agents.</p>
 866
 867 <div class="good">
 868 <p><strong><em>CORRECT: terminated empty tags</em></strong></p>
 869
 870 <p>&lt;br/&gt;&lt;hr/&gt;</p>
 871 </div>
 872
 873 <div class="bad">
 874 <p><strong><em>INCORRECT: unterminated empty tags</em></strong></p>
 875
 876 <p>&lt;br&gt;&lt;hr&gt;</p>
 877 </div>
 878
 879 <h2><a name="h-4.7" id="h-4.7">4.7 Whitespace handling in
 880 attribute values</a></h2>
 881
 882 <p>In attribute values, user agents will strip leading and
 883 trailing whitespace from attribute values and map sequences
 884 of one or more whitespace characters (including line breaks) to
 885 a single inter-word space (an ASCII space character for western
 886 scripts). See <a href="http://www.w3.org/TR/REC-xml#AVNormalize">
 887 Section 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
 890 elements</a></h2>
 891
 892 <p>In XHTML, the script and style elements are declared as having
 893 <code>#PCDATA</code> content. As a result, <code>&lt;</code> and
 894 <code>&amp;</code> will be treated as the start of markup, and
 895 entities such as <code>&amp;lt;</code> and <code>&amp;amp;</code>
 896 will be recognized as entity references by the XML processor to
 897 <code>&lt;</code> and <code>&amp;</code> respectively. Wrapping
 898 the content of the script or style element within a <code>
 899 CDATA</code> marked section avoids the expansion of these
 900 entities.</p>
 901
 902 <div class="good">
 903 <pre>
 904 &lt;script&gt;
 905  &lt;![CDATA[
 906  ... unescaped script content ...
 907  ]]&gt;
 908  &lt;/script&gt;
 909 </pre>
 910 </div>
 911
 912 <p><code>CDATA</code> sections are recognized by the XML
 913 processor 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">
 915 Section 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
 918 documents.</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
 923 elements from being contained within an element. Such
 924 prohibitions (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
 928 to any descendant depth. It is not possible to spell out such
 929 prohibitions in XML. Even though these prohibitions cannot be
 930 defined in the DTD, certain elements should not be nested. A
 931 summary of such elements and the elements that should not be
 932 nested in them is found in the normative <a href="#prohibitions">
 933 Appendix&#xA0;B</a>.</p>
 934
 935 <h2><a name="h-4.10" id="h-4.10">4.10 The elements with 'id' and 'name'
 936 attributes</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>.
 942 HTML 4.0 also introduced
 943 the <code>id</code> attribute. Both of these attributes are designed to be
 944 used as fragment identifiers.</p>
 945 <p>In XML, fragment identifiers are of type <code>ID</code>, and
 946 there can only be a single attribute of type <code>ID</code> per element.
 947 Therefore, in XHTML 1.0 the <code>id</code>
 948 attribute is defined to be of type <code>ID</code>. In order to
 949 ensure that XHTML 1.0 documents are well-structured XML documents, XHTML 1.0
 950 documents MUST use the <code>id</code> attribute when defining fragment
 951 identifiers, even on elements that historically have also had a
 952 <code>name</code> attribute.
 953 See the <a href="#guidelines">HTML Compatibility
 954 Guidelines</a> for information on ensuring such anchors are backwards
 955 compatible 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
 958 elements is formally deprecated, and will be removed in a
 959 subsequent 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
 965 compatible with existing user agents, in practice this is easy to
 966 accomplish. Guidelines for creating compatible documents can be
 967 found in <a href="#guidelines">Appendix&#xA0;C</a>.</p>
 968
 969 <h2><a name="media" id="media">5.1 Internet Media Type</a></h2>
 970 <p>As of the publication of this recommendation, the general
 971 recommended MIME labeling for XML-based applications
 972 has yet to be resolved.</p>
 973
 974 <p>However, XHTML Documents which follow the guidelines set forth
 975 in <a href="#guidelines">Appendix C</a>, "HTML Compatibility Guidelines" may be
 976 labeled with the Internet Media Type "text/html", as they
 977 are compatible with most HTML browsers. This document
 978 makes no recommendation about MIME labeling of other
 979 XHTML 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
 985 that will extend and subset XHTML, in order to support a wide
 986 range of new devices and applications, by defining modules and
 987 specifying a mechanism for combining these modules. This
 988 mechanism will enable the extension and sub-setting of XHTML 1.0
 989 in 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
 994 agents to other platforms, it is clear that not all of the XHTML
 995 elements will be required on all platforms. For example a hand
 996 held device or a cell-phone may only support a subset of XHTML
 997 elements.</p>
 998
 999 <p>The process of modularization breaks XHTML up into a series of
1000 smaller element sets. These elements can then be recombined to
1001 meet 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
1006 Extensibility</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
1029 Profiles</a></h2>
1030
1031 <p>A document profile specifies the syntax and semantics of a set
1032 of documents. Conformance to a document profile provides a basis
1033 for interoperability guarantees. The document profile specifies
1034 the facilities required to process documents of that type, e.g.
1035 which image formats can be used, levels of scripting, style sheet
1036 support, and so on.</p>
1037
1038 <p>For product designers this enables various groups to define
1039 their own standard profile.</p>
1040
1041 <p>For authors this will obviate the need to write several
1042 different versions of documents for different clients.</p>
1043
1044 <p>For special groups such as chemists, medical doctors, or
1045 mathematicians this allows a special profile to be built using
1046 standard HTML elements plus a group of elements geared to the
1047 specialist'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
1056 specification. The complete set of DTD files together with an XML
1057 declaration 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
1060 Definitions</a></h2>
1061
1062 <p>These DTDs approximate the HTML 4.0 DTDs. It is likely that
1063 when the DTDs are modularized, a method of DTD construction will
1064 be 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">
1069 XHTML-1.0-Strict</a></p>
1070 </li>
1071
1072 <li>
1073 <p><a href="DTD/xhtml1-transitional.dtd" type="text/plain">
1074 XHTML-1.0-Transitional</a></p>
1075 </li>
1076
1077 <li>
1078 <p><a href="DTD/xhtml1-frameset.dtd" type="text/plain">
1079 XHTML-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
1086 been modified to be valid XML 1.0 entity declarations. Note the
1087 entity for the Euro currency sign (<code>&amp;euro;</code> or
1088 <code>&amp;#8364;</code> or <code>&amp;#x20AC;</code>) is defined
1089 as 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
1107 Prohibitions</a></h1>
1108
1109 <p><b>This appendix is normative.</b></p>
1110
1111 <p>The following elements have prohibitions on which elements
1112 they can contain (see <a href="#h-4.9">Section 4.9</a>). This
1113 prohibition applies to all depths of nesting, i.e. it contains
1114 all the descendant elements.</p>
1115
1116 <dl><dt><code class="tag">a</code></dt>
1117 <dd>
1118 cannot 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>
1122 sup</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.
1137 HTML 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
1142 wish their XHTML documents to render on existing HTML user
1143 agents.</p>
1144
1145 <h2>C.1 Processing Instructions</h2>
1146 <p>Be aware that processing instructions are rendered on some
1147 user agents. However, also note that when the XML declaration is not included
1148 in a document, the document can only use the default character encodings UTF-8
1149 or UTF-16.</p>
1150
1151 <h2>C.2 Empty Elements</h2>
1152 <p>Include a space before the trailing <code>/</code> and <code>
1153 &gt;</code> of empty elements, e.g. <code class="greenmono">
1154 &lt;br&#xA0;/&gt;</code>, <code class="greenmono">
1155 &lt;hr&#xA0;/&gt;</code> and <code class="greenmono">&lt;img
1156 src="karen.jpg" alt="Karen"&#xA0;/&gt;</code>. Also, use the
1157 minimized tag syntax for empty elements, e.g. <code class="greenmono">&lt;br /&gt;</code>, as the alternative syntax <code class="greenmono">&lt;br&gt;&lt;/br&gt;</code> allowed by XML
1158 gives 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
1162 not <code>EMPTY</code> (for example, an empty title or paragraph)
1163 do not use the minimized form (e.g. use <code class="greenmono">
1164 &lt;p&gt; &lt;/p&gt;</code> and not <code class="greenmono">
1165 &lt;p&#xA0;/&gt;</code>).</p>
1166
1167 <h2>C.4 Embedded Style Sheets and Scripts</h2>
1168 <p>Use external style sheets if your style sheet uses <code>
1169 &lt;</code> or <code>&amp;</code> or <code>]]&gt;</code> or <code>--</code>. Use
1170 external scripts if your script uses <code>&lt;</code> or <code>
1171 &amp;</code> or <code>]]&gt;</code> or <code>--</code>. Note that XML parsers
1172 are permitted to silently remove the contents of comments. Therefore, the historical
1173 practice of "hiding" scripts and style sheets within comments to make the
1174 documents backward compatible is likely to not work as expected in XML-based
1175 implementations.</p>
1176
1177 <h2>C.5 Line Breaks within Attribute Values</h2>
1178 <p>Avoid line breaks and multiple whitespace characters within
1179 attribute values. These are handled inconsistently by user
1180 agents.</p>
1181
1182 <h2>C.6 Isindex</h2>
1183 <p>Don't include more than one <code>isindex</code> element in
1184 the document <code>head</code>. The <code>isindex</code> element
1185 is 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>
1189 attributes when specifying the language of an element. The value
1190 of 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
1196 attribute defined to be of type <code>ID</code>, e.g., the <code>
1197 id</code> attribute in HTML 4.0. Many existing HTML clients don't
1198 support the use of <code>ID</code>-type attributes in this way,
1199 so identical values may be supplied for both of these attributes to ensure
1200 maximum forward and backward compatibility (e.g., <code class="greenmono">&lt;a id="foo" name="foo"&gt;...&lt;/a&gt;</code>).</p>
1201
1202 <p>Further, since the set of
1203 legal values for attributes of type <code>ID</code> is much smaller than
1204 for those of type <code>CDATA</code>, the type of the <code>name</code>
1205 attribute has been changed to <code>NMTOKEN</code>. This attribute is
1206 constrained 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
1208 2.5, production 5. Unfortunately, this constraint cannot be expressed in the
1209 XHTML 1.0 DTDs.  Because of this change, care must be taken when
1210 converting existing HTML documents. The values of these attributes
1211 must be unique within the document, valid, and any references to these
1212 fragment identifiers (both
1213 internal and external) must be updated should the values be changed during
1214 conversion.</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>
1217 elements, and it will be
1218 removed 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
1222 encoding attribute specification on the xml declaration (e.g.
1223 <code class="greenmono">&lt;?xml version="1.0"
1224 encoding="EUC-JP"?&gt;</code>) and a meta http-equiv statement
1225 (e.g. <code class="greenmono">&lt;meta http-equiv="Content-type"
1226 content='text/html; charset="EUC-JP"'&#xA0;/&gt;</code>). The
1227 value of the encoding attribute of the xml processing instruction
1228 takes precedence.</p>
1229
1230 <h2>C.10 Boolean Attributes</h2>
1231 <p>Some HTML user agents are unable to interpret boolean
1232 attributes when these appear in their full (non-minimized) form,
1233 as required by XML 1.0. Note this problem doesn't effect user
1234 agents compliant with HTML 4.0. The following attributes are
1235 involved: <code>compact</code>, <code>nowrap</code>, <code>
1236 ismap</code>, <code>declare</code>, <code>noshade</code>, <code>
1237 checked</code>, <code>disabled</code>, <code>readonly</code>,
1238 <code>multiple</code>, <code>selected</code>, <code>
1239 noresize</code>, <code>defer</code>.</p>
1240
1241 <h2>C.11 Document Object Model and XHTML</h2>
1242 <p>
1243 The Document Object Model level 1 Recommendation [<a href="#ref-dom">DOM</a>]
1244 defines document object model interfaces for XML and HTML 4.0. The HTML 4.0
1245 document object model specifies that HTML element and attribute names are
1246 returned in upper-case. The XML document object model specifies that
1247 element and attribute names are returned in the case they are specified. In
1248 XHTML 1.0, elements and attributes are specified in lower-case. This apparent difference can be
1249 addressed in two ways:
1250 </p>
1251 <ol>
1252 <li>Applications that access XHTML documents served as Internet media type
1253 <code>text/html</code>
1254 via the <abbr title="Document Object Model">DOM</abbr> can use the HTML DOM,
1255 and can rely upon element and attribute names being returned in
1256 upper-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>
1259 can also use the XML DOM. Elements and attributes will be returned in lower-case.
1260 Also, some XHTML elements may or may
1261 not appear
1262 in 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
1265 permitted to be minimized such that their start and end tags are both omitted
1266 (an SGML feature).
1267 This is not possible in XML. Rather than require document authors to insert
1268 extraneous elements, XHTML has made the elements optional.
1269 Applications need to adapt to this
1270 accordingly.</li>
1271 </ol>
1272
1273 <h2>C.12 Using Ampersands in Attribute Values</h2>
1274 <p>
1275 When an attribute value contains an ampersand, it must be expressed as a character
1276 entity reference
1277 (e.g. "<code>&amp;amp;</code>"). For example, when the
1278 <code>href</code> attribute
1279 of the <code>a</code> element refers to a
1280 CGI script that takes parameters, it must be expressed as
1281 <code>http://my.site.dom/cgi-bin/myscript.pl?class=guest&amp;amp;name=user</code>
1282 rather than as
1283 <code>http://my.site.dom/cgi-bin/myscript.pl?class=guest&amp;name=user</code>.
1284 </p>
1285
1286 <h2>C.13 Cascading Style Sheets (CSS) and XHTML</h2>
1287
1288 <p>The Cascading Style Sheets level 2 Recommendation [<a href="#ref-css2">CSS2</a>] defines style
1289 properties which are applied to the parse tree of the HTML or XML
1290 document.  Differences in parsing will produce different visual or
1291 aural results, depending on the selectors used. The following hints
1292 will reduce this effect for documents which are served without
1293 modification as both media types:</p>
1294
1295 <ol>
1296 <li>
1297 CSS style sheets for XHTML should use lower case element and
1298 attribute names.</li>
1299
1300
1301 <li>In tables, the tbody element will be inferred by the parser of an
1302 HTML user agent, but not by the parser of an XML user agent. Therefore
1303 you should always explicitely add a tbody element if it is referred to
1304 in a CSS selector.</li>
1305
1306 <li>Within the XHTML name space, user agents are expected to
1307 recognize the "id" attribute as an attribute of type ID.
1308 Therefore, style sheets should be able to continue using the
1309 shorthand "#" selector syntax even if the user agent does not read
1310 the DTD.</li>
1311
1312 <li>Within the XHTML name space, user agents are expected to
1313 recognize the "class" attribute. Therefore, style sheets should be
1314 able to continue using the shorthand "." selector syntax.</li>
1315
1316 <li>
1317 CSS defines different conformance rules for HTML and XML documents;
1318 be aware that the HTML rules apply to XHTML documents delivered as
1319 HTML 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.
1323 Acknowledgements</a></h1>
1324
1325 <p><b>This appendix is informative.</b></p>
1326
1327 <p>This specification was written with the participation of the
1328 members of the W3C HTML working group:</p>
1329
1330 <dl>
1331 <dd>Steven Pemberton, CWI (HTML Working Group Chair)<br />
1332 Murray Altheim, Sun Microsystems<br />
1333 Daniel Austin, CNET: The Computer Network<br />
1334 Frank Boumphrey, HTML Writers Guild<br />
1335 John Burger, Mitre<br />
1336 Andrew W. Donoho, IBM<br />
1337 Sam Dooley, IBM<br />
1338 Klaus Hofrichter, GMD<br />
1339 Philipp Hoschka, W3C<br />
1340 Masayasu Ishikawa, W3C<br />
1341 Warner ten Kate, Philips Electronics<br />
1342 Peter King, Phone.com<br />
1343 Paula Klante, JetForm<br />
1344 Shin'ichi Matsui, W3C/Panasonic<br />
1345 Shane McCarron, Applied Testing and Technology (The Open Group through August
1346 1999)<br />
1347 Ann Navarro, HTML Writers Guild<br />
1348 Zach Nies, Quark<br />
1349 Dave Raggett, W3C/HP (W3C lead for HTML)<br />
1350 Patrick Schmitz, Microsoft<br />
1351 Sebastian Schnitzenbaumer, Stack Overflow<br />
1352 Chris Wilson, Microsoft<br />
1353 Ted Wugofski, Gateway 2000<br />
1354 Dan 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.
1367 Bos, H. W. Lie, C. Lilley, I. Jacobs, 12 May 1998.<br />
1368 Available at: <a href="http://www.w3.org/TR/REC-CSS2">
1369 http://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
1374 Wood <i>et al.</i>, 1 October 1998.<br />
1375 Available at: <a href="http://www.w3.org/TR/REC-DOM-Level-1">
1376 http://www.w3.org/TR/REC-DOM-Level-1</a></dd>
1377
1378 <dt><a name="ref-html4" id="ref-html4"><b>[HTML]</b></a></dt>
1379
1380 <dd><a href="http://www.w3.org/TR/1999/PR-html40-19990824">"HTML 4.01 Specification"</a>, D. Raggett, A. Le&#xA0;Hors, I.
1381 Jacobs, 24 August 1999.<br />
1382 Available at: <a href="http://www.w3.org/TR/1999/PR-html40-19990824">
1383 http://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
1388 Operating System Interface (POSIX) - Part 1: System Application
1389 Program Interface (API) [C Language]", Institute of Electrical
1390 and 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
1396 Two: Media Types"</a>, N. Freed and N. Borenstein, November
1397 1996.<br />
1398 Available at <a href="http://www.ietf.org/rfc/rfc2046.txt">
1399 http://www.ietf.org/rfc/rfc2046.txt</a>. Note that this RFC
1400 obsoletes 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
1406 Levels"</a>, S. Bradner, March 1997.<br />
1407 Available at: <a href="http://www.ietf.org/rfc/rfc2119.txt">
1408 http://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
1414 1998.<br />
1415 Available at: <a href="http://www.ietf.org/rfc/rfc2376.txt">
1416 http://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
1422 Syntax"</a>, T. Berners-Lee, R. Fielding, L. Masinter, August
1423 1998.<br />
1424 This document updates RFC1738 and RFC1808.<br />
1425 Available at: <a href="http://www.ietf.org/rfc/rfc2396.txt">
1426 http://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.
1431 Bray, J. Paoli, C. M. Sperberg-McQueen, 10 February 1998.<br />
1432 Available at: <a href="http://www.w3.org/TR/REC-xml">
1433 http://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
1438 January 1999.<br />
1439 XML namespaces provide a simple method for qualifying names used
1440 in XML documents by associating them with namespaces identified
1441 by URI.<br />
1442 Available at: <a href="http://www.w3.org/TR/REC-xml-names">
1443 http://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>