Added UAAG reference for access to longdescs - http://lists.w3.org/Archives/Public/w3c-wai-ig/2013AprJun/0002.html
authorcharles
Wed, 03 Apr 2013 14:26:03 +0200
changeset 29 88d14b8800fd
parent 28 753af00ced01
child 30 0dd2e510d4e1
Added UAAG reference for access to longdescs - http://lists.w3.org/Archives/Public/w3c-wai-ig/2013AprJun/0002.html

Added WCAG reference for content of longdesc targets - https://www.w3.org/Bugs/Public/show_bug.cgi?id=21437

Clarified that attribute value is a valid *non-empty* URL… - https://www.w3.org/Bugs/Public/show_bug.cgi?id=21436

Split informative advice to sections matching the "audience" for normative requirements - https://www.w3.org/Bugs/Public/show_bug.cgi?id=21438

Added examples showing use of data: URIs - https://www.w3.org/Bugs/Public/show_bug.cgi?id=21439

Added example of repairing text descriptions by converting them to data: URIs - https://www.w3.org/Bugs/Public/show_bug.cgi?id=21439
longdesc1/longdesc.html
--- a/longdesc1/longdesc.html	Fri Mar 22 02:42:09 2013 +0100
+++ b/longdesc1/longdesc.html	Wed Apr 03 14:26:03 2013 +0200
@@ -1,4 +1,4 @@
-<!DOCTYPE html>
<html dir="ltr" lang="en">
  <head>
    <meta charset="UTF-8">
    <title>HTML Image Description Extension</title>
    <style>
      .example, .example code {color: #404; background-color:#eee}
      .example::before {float:left;
        margin-top: -.2em;
        background-color: white;
            display: block;
            border: 1px solid black;
            color: #404;
            padding: 2px;
        content: "Example: (non-normative)"}
      
      .example:first-child {padding-top:1em}
      
        /* Make these stand-out more... */
        .externalDFN {
            font-style: italic;
            background-color: #fff9d6;
        }
        /* Switch statement */
        dl.switch dt::before {
            content: "↪ ";
            margin-left: 1em;
        }
        /* Better spacing around various lists (implied paragraph children) */
        ol > li, section:not(#toc) ul > li, section dl > dt {
            margin: 1em 0;
        }
        /* domintro styling */
        dl.domintro {
            background-color: rgb(221, 255, 221);
            padding: 1em 0.5em 1em 2em;
            clear: both;
        }
        dl.domintro dt {
            color: black;
        }
        dl.domintro > dd {
            color: green;
        }
        dl.domintro::before {
            float: right;
            background-color: white;
            display: block;
            border: 2px solid black;
            color: green;
            margin-top: -20px;
            padding: 2px;
            content: "This box is non-normative.";
        }
        /* Fancy table stuff */
        table {
            border-collapse: collapse;
        }
        thead tr {
            border-bottom: 2px solid black;
        }
        tbody tr:not(:last-child) {
            border-bottom: 1px solid black;
        }
        td {
            border-left: 1px solid black;
            padding: 4px;
        }

/*****************************************************************
 * ReSpec 3 CSS
 * Robin Berjon - http://berjon.com/
 *****************************************************************/

/* --- INLINES --- */
em.rfc2119 { 
    text-transform:     lowercase;
    font-variant:       small-caps;
    font-style:         normal;
    color:              #900;
}

h1 acronym, h2 acronym, h3 acronym, h4 acronym, h5 acronym, h6 acronym, a acronym,
h1 abbr, h2 abbr, h3 abbr, h4 abbr, h5 abbr, h6 abbr, a abbr {
    border: none;
}

dfn {
    font-weight:    bold;
}

a.internalDFN {
    color:  inherit;
    border-bottom:  1px solid #99c;
    text-decoration:    none;
}

a.externalDFN {
    color:  inherit;
    border-bottom:  1px dotted #ccc;
    text-decoration:    none;
}

a.bibref {
    text-decoration:    none;
}

cite .bibref {
    font-style: normal;
}

code {
    color:  #ff4500;
}

/* --- TOC --- */
.toc a, .tof a {
    text-decoration:    none;
}

a .secno, a .figno {
    color:  #000;
}

ul.tof, ol.tof {
    list-style: none outside none;
}

.caption {
    margin-top: 0.5em;
    font-style:   italic;
}


/* --- DL --- */
.section dd > p:first-child {
    margin-top: 0;
}

.section dd > p:last-child {
    margin-bottom: 0;
}

.section dd {
    margin-bottom:  1em;
}

.section dl.attrs dd, .section dl.eldef dd {
    margin-bottom:  0;
}
/* --- ISSUES/NOTES --- */
div.note-title {
    padding-right:  1em;
    min-width: 7.5em;
    color: #b9ab2d;
}

div.note-title { color: #52e052; }
div.note-title span {
    text-transform: uppercase;
}
div.note {
    margin-top: 1em;
    margin-bottom: 1em;
}
.note > p:first-child { margin-top: 0 }
.note {
    padding: .5em;
    border-left-width: .5em;
    border-left-style: solid;
}
div.note {
    padding: 0.5em;
    margin: 1em 0;
    position: relative;
    clear: both;
}
span.note { padding: .1em .5em .15em; }

.note {
    border-color: #52e052;
    background: #e9fbe9;
}


pre.idl {
    border-top: 1px solid #90b8de;
    border-bottom: 1px solid #90b8de;
    padding:    1em;
    line-height:    120%;
}

pre.idl::before {
    content:    "WebIDL";
    display:    block;
    width:      6ex;
    background: #90b8de;
    color:  #fff;
    font-family:    initial;
    padding:    3px;
    font-weight:    bold;
    margin: -1em 0 1em -1em;
}

.idlType {
    color:  #ff4500;
    font-weight:    bold;
    text-decoration:    none;
}

/*.idlInterface*/
.idlInterfaceID, .idlDictionaryID, .idlCallbackID, .idlEnumID {
    font-weight:    bold;
    color:  #005a9c;
}

.idlSuperclass {
    font-style: italic;
    color:  #005a9c;
}

/*.idlAttribute*/
.idlAttrType, .idlFieldType, .idlMemberType {
    color:  #005a9c;
}
.idlAttrName, .idlFieldName, .idlMemberName {
    color:  #ff4500;
}
.idlAttrName a, .idlFieldName a, .idlMemberName a {
    color:  #ff4500;
    border-bottom:  1px dotted #ff4500;
    text-decoration: none;
}



.extAttr {
    color:  #666;
}

/*.idlConst*/
.idlConstType {
    color:  #005a9c;
}
.idlConstName {
    color:  #ff4500;
}
.idlConstName a {
    color:  #ff4500;
    border-bottom:  1px dotted #ff4500;
    text-decoration: none;
}


.idlImplements a {
    font-weight:    bold;
}

dl.attributes, dl.methods, dl.constants, dl.fields, dl.dictionary-members {
    margin-left:    2em;
}

.attributes dt, .methods dt, .constants dt, .fields dt, .dictionary-members dt {
    font-weight:    normal;
}

.attributes dt code, .methods dt code, .constants dt code, .fields dt code, .dictionary-members dt code {
    font-weight:    bold;
    color:  #000;
    font-family:    monospace;
}

.attributes dt code, .fields dt code, .dictionary-members dt code {
    background:  #ffffd2;
}

.attributes dt .idlAttrType code, .fields dt .idlFieldType code, .dictionary-members dt .idlMemberType code {
    color:  #005a9c;
    background:  transparent;
    font-family:    inherit;
    font-weight:    normal;
    font-style: italic;
}



.attributes dd, .methods dd, .constants dd, .fields dd, .dictionary-members dd {
    margin-bottom:  1em;
}



</style> <link href="https://www.w3.org/StyleSheets/TR/W3C-ED" rel="stylesheet">
    <!--[if lt IE 9]><script src='undefined://www.w3.org/2008/site/js/html5shiv.js'></script><![endif]-->
  </head>
  <body>
    <div class="head">
      <p> <a href="http://www.w3.org/"><img alt="W3C" src="http://www.w3.org/Icons/w3c_home"
            height="48"
            width="72"></a>
      </p>
      <h1 class="title" id="title">HTML5 Image Description Extension </h1>
      <h2 id="w3c-working-draft-20-september-2012"><abbr title="World Wide Web Consortium">W3C</abbr>
        Editors' Draft 22 March 2013</h2>
      <dl>
        <dt>Latest published version:</dt>
        <dd><a href="http://www.w3.org/TR/HTML-longdesc">http://www.w3.org/TR/HTML-longdesc</a></dd>
        <dt>Latest editor's draft:</dt>
        <dd> <a href="http://dvcs.w3.org/hg/html-proposals/raw-file/default/longdesc1/longdesc.html">http://dvcs.w3.org/hg/html-proposals/raw-file/default/longdesc1/longdesc.html</a></dd>
        <dt>Editors:</dt>
        <dd><a href="mailto:[email protected]">Charles McCathie Nevile</a>,
          <a href="http://www.yandex.ru">Яндекс</a>--<a href="http://yandex.com">Yandex</a></dd>
      </dl>
      <p class="copyright"> <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>
        © 2012-2013 <a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup>
        (<a href="http://www.csail.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>,
        <a href="http://www.ercim.eu/"><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
        <a href="http://www.keio.ac.jp/">Keio</a>, <a href="http://ev.buaa.edu.cn/">Beihang</a>),
+<!DOCTYPE html>
<html dir="ltr" lang="en">
  <head>
    <meta content="text/html; charset=UTF-8" http-equiv="content-type">
    <title>HTML Image Description Extension</title>
    <style>
      .example, .example code {color: #404; background-color:#eee}
      .example::before {float:left;
        margin-top: -.2em;
        background-color: white;
            display: block;
            border: 1px solid black;
            color: #404;
            padding: 2px;
        content: "Example: (non-normative)"}
      
      .example:first-child {padding-top:1em}
      
        /* Make these stand-out more... */
        .externalDFN {
            font-style: italic;
            background-color: #fff9d6;
        }
        /* Switch statement */
        dl.switch dt::before {
            content: "↪ ";
            margin-left: 1em;
        }
        /* Better spacing around various lists (implied paragraph children) */
        ol > li, section:not(#toc) ul > li, section dl > dt {
            margin: 1em 0;
        }
        /* domintro styling */
        dl.domintro {
            background-color: rgb(221, 255, 221);
            padding: 1em 0.5em 1em 2em;
            clear: both;
        }
        dl.domintro dt {
            color: black;
        }
        dl.domintro > dd {
            color: green;
        }
        dl.domintro::before {
            float: right;
            background-color: white;
            display: block;
            border: 2px solid black;
            color: green;
            margin-top: -20px;
            padding: 2px;
            content: "This box is non-normative.";
        }
        /* Fancy table stuff */
        table {
            border-collapse: collapse;
        }
        thead tr {
            border-bottom: 2px solid black;
        }
        tbody tr:not(:last-child) {
            border-bottom: 1px solid black;
        }
        td {
            border-left: 1px solid black;
            padding: 4px;
        }

/*****************************************************************
 * ReSpec 3 CSS
 * Robin Berjon - http://berjon.com/
 *****************************************************************/

/* --- INLINES --- */
em.rfc2119 { 
    text-transform:     lowercase;
    font-variant:       small-caps;
    font-style:         normal;
    color:              #900;
}

h1 acronym, h2 acronym, h3 acronym, h4 acronym, h5 acronym, h6 acronym, a acronym,
h1 abbr, h2 abbr, h3 abbr, h4 abbr, h5 abbr, h6 abbr, a abbr {
    border: none;
}

dfn {
    font-weight:    bold;
}

a.internalDFN {
    color:  inherit;
    border-bottom:  1px solid #99c;
    text-decoration:    none;
}

a.externalDFN {
    color:  inherit;
    border-bottom:  1px dotted #ccc;
    text-decoration:    none;
}

a.bibref {
    text-decoration:    none;
}

cite .bibref {
    font-style: normal;
}

code {
    color:  #ff4500;
}

/* --- TOC --- */
.toc a, .tof a {
    text-decoration:    none;
}

a .secno, a .figno {
    color:  #000;
}

ul.tof, ol.tof {
    list-style: none outside none;
}

.caption {
    margin-top: 0.5em;
    font-style:   italic;
}


/* --- DL --- */
.section dd > p:first-child {
    margin-top: 0;
}

.section dd > p:last-child {
    margin-bottom: 0;
}

.section dd {
    margin-bottom:  1em;
}

.section dl.attrs dd, .section dl.eldef dd {
    margin-bottom:  0;
}
/* --- ISSUES/NOTES --- */
div.note-title {
    padding-right:  1em;
    min-width: 7.5em;
    color: #b9ab2d;
}

div.note-title { color: #52e052; }
div.note-title span {
    text-transform: uppercase;
}
div.note {
    margin-top: 1em;
    margin-bottom: 1em;
}
.note > p:first-child { margin-top: 0 }
.note {
    padding: .5em;
    border-left-width: .5em;
    border-left-style: solid;
}
div.note {
    padding: 0.5em;
    margin: 1em 0;
    position: relative;
    clear: both;
}
span.note { padding: .1em .5em .15em; }

.note {
    border-color: #52e052;
    background: #e9fbe9;
}


pre.idl {
    border-top: 1px solid #90b8de;
    border-bottom: 1px solid #90b8de;
    padding:    1em;
    line-height:    120%;
}

pre.idl::before {
    content:    "WebIDL";
    display:    block;
    width:      8ex;
    background: #90b8de;
    color:  #fff;
    font-family:    initial;
    padding:    3px;
    font-weight:    bold;
    margin: -1em 0 1em -1em;
}

.idlType {
    color:  #ff4500;
    font-weight:    bold;
    text-decoration:    none;
}

/*.idlInterface*/
.idlInterfaceID, .idlDictionaryID, .idlCallbackID, .idlEnumID {
    font-weight:    bold;
    color:  #005a9c;
}

.idlSuperclass {
    font-style: italic;
    color:  #005a9c;
}

/*.idlAttribute*/
.idlAttrType, .idlFieldType, .idlMemberType {
    color:  #005a9c;
}
.idlAttrName, .idlFieldName, .idlMemberName {
    color:  #ff4500;
}
.idlAttrName a, .idlFieldName a, .idlMemberName a {
    color:  #ff4500;
    border-bottom:  1px dotted #ff4500;
    text-decoration: none;
}



.extAttr {
    color:  #666;
}

/*.idlConst*/
.idlConstType {
    color:  #005a9c;
}
.idlConstName {
    color:  #ff4500;
}
.idlConstName a {
    color:  #ff4500;
    border-bottom:  1px dotted #ff4500;
    text-decoration: none;
}


.idlImplements a {
    font-weight:    bold;
}

dl.attributes, dl.methods, dl.constants, dl.fields, dl.dictionary-members {
    margin-left:    2em;
}

.attributes dt, .methods dt, .constants dt, .fields dt, .dictionary-members dt {
    font-weight:    normal;
}

.attributes dt code, .methods dt code, .constants dt code, .fields dt code, .dictionary-members dt code {
    font-weight:    bold;
    color:  #000;
    font-family:    monospace;
}

.attributes dt code, .fields dt code, .dictionary-members dt code {
    background:  #ffffd2;
}

.attributes dt .idlAttrType code, .fields dt .idlFieldType code, .dictionary-members dt .idlMemberType code {
    color:  #005a9c;
    background:  transparent;
    font-family:    inherit;
    font-weight:    normal;
    font-style: italic;
}



.attributes dd, .methods dd, .constants dd, .fields dd, .dictionary-members dd {
    margin-bottom:  1em;
}



</style> <link href="https://www.w3.org/StyleSheets/TR/W3C-ED" rel="stylesheet">
    <!--[if lt IE 9]><script src='undefined://www.w3.org/2008/site/js/html5shiv.js'></script><![endif]-->
  </head>
  <body>
    <div class="head">
      <p> <a href="http://www.w3.org/"><img alt="W3C" src="http://www.w3.org/Icons/w3c_home"
            height="48"
            width="72"></a>
      </p>
      <h1 class="title" id="title">HTML5 Image Description Extension </h1>
      <h2 id="w3c-working-draft-20-september-2012"><abbr title="World Wide Web Consortium">W3C</abbr>
        Editors' Draft 2 April 2013</h2>
      <dl>
        <dt>Latest published version:</dt>
        <dd><a href="http://www.w3.org/TR/HTML-longdesc">http://www.w3.org/TR/HTML-longdesc</a></dd>
        <dt>Latest editor's draft:</dt>
        <dd> <a href="http://dvcs.w3.org/hg/html-proposals/raw-file/default/longdesc1/longdesc.html">http://dvcs.w3.org/hg/html-proposals/raw-file/default/longdesc1/longdesc.html</a></dd>
        <dt>Editors:</dt>
        <dd><a href="mailto:[email protected]">Charles McCathie Nevile</a>,
          <a href="http://www.yandex.ru">Яндекс</a>--<a href="http://yandex.com">Yandex</a></dd>
      </dl>
      <p class="copyright"> <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>
        © 2012-2013 <a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup>
        (<a href="http://www.csail.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>,
        <a href="http://www.ercim.eu/"><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
        <a href="http://www.keio.ac.jp/">Keio</a>, <a href="http://ev.buaa.edu.cn/">Beihang</a>),
         All Rights Reserved. <abbr title="World Wide Web Consortium">W3C</abbr>
        <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
        <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
        and <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document
           use</a> rules apply. </p>
      <hr> </div>
    <section class="introductory" id="abstract">
      <h2>Abstract</h2>
      <p>This specification defines a <code>longdesc</code> attribute to link
        extended descriptions with images in HTML5-based content. </p>
    </section>
    <section class="introductory" id="sotd">
      <h2>Status of This Document</h2>
      <p> <em>This section describes the status of this document at the time of
          its publication. Other documents may supersede this document. A list
          of current <abbr title="World Wide Web Consortium">W3C</abbr>
          publications and the latest revision of this technical report can be
          found in the <a href="http://www.w3.org/TR/"><abbr title="World Wide Web Consortium">W3C</abbr>
            technical reports index</a> at http://www.w3.org/TR/.</em> </p>
      <p> This document is an editors' draft proposed to the HTML Working Group
        Accessibility Task Force following the First Public Working Draft of an
        HTML extension Proposal. This document is intended to become a <abbr title="World Wide Web Consortium">W3C</abbr>
        Recommendation, either on its own or merged into an HTML Recommendation.
        If you wish to make comments regarding this document, please send them
        to <a href="mailto:[email protected]">[email protected]</a>
        (<a href="mailto:[email protected]?subject=subscribe">subscribe</a>,
        <a href="http://lists.w3.org/Archives/Public/public-html-a11y/">archives</a>).
         All feedback is welcome. Bugs can also be filed directly into the <a href="https://www.w3.org/Bugs/Public/buglist.cgi?product=HTML%20WG&amp;amp;component=HTML%20Image%20Description%20Extension">W3C
@@ -15,10 +15,10 @@
               </span>Acknowledgements</a></li>
        <li class="tocline"><a class="tocxref" href="#references"><span class="secno">B.
               </span>References</a>
          <ul class="toc">
            <li class="tocline"><a class="tocxref" href="#normative-references"><span
                  class="secno">B.1
                   </span>Normative references</a></li>
            <li class="tocline"><a class="tocxref" href="#informative-references"><span
                  class="secno">B.2
-                  </span>Informative references</a></li>
          </ul>
        </li>
      </ul>
    </section>
    <section id="intro" class="introductory">
      <h1>Introduction</h1>
      <p>HTML 4 provided a <code>longdesc</code> attribute for the <code>img</code>
        element. This enabled a machine-discoverable description of an image to
        be linked to the image. This extension specification defines the same
        functionality for HTML5-based content.</p>
      <pre class="example"><code><br><br>&lt;!-- The description is somewhere on the same page as the image --&gt;
&lt;img src="http://example.com/graph1" alt="(The trend is for drinks to get sweeter)" title="Figure One" <strong>longdesc="#graph1"</strong>&gt;

&lt;!-- The description is a page on its own --&gt;
&lt;img src="figure1" alt="figure 1" <strong>longdesc="http://example.com/desc1"</strong>&gt;

&lt;!-- The description is one of several within an external page --&gt;
&lt;img src="ExampleImage" alt="example" <strong>longdesc="http://example.com/descs#item3"</strong>&gt;</code></pre>
    </section>
    <section id="conformance">
      <h2><span class="secno">1. </span>Conformance</h2>
      <p> As well as sections marked as non-normative, all authoring guidelines,
        diagrams, examples, and notes in this specification are non-normative.
        Everything else in this specification is normative. </p>
      <p> The key words <em title="must" class="rfc2119">must</em>, <em title="should"
          class="rfc2119">should</em>,
        <em title="should not" class="rfc2119"></em><em title="recommended" class="rfc2119"></em>and
+                  </span>Informative references</a></li>
          </ul>
        </li>
      </ul>
    </section>
    <section id="intro" class="introductory">
      <h1>Introduction</h1>
      <p>HTML 4 provided a <code>longdesc</code> attribute for the <code>img</code>
        element. This enabled a machine-discoverable description of an image to
        be linked to the image. This extension specification defines the same
        functionality for HTML5-based content.</p>
      <pre class="example"><code><br><br>&lt;!-- The description is somewhere on the same page as the image --&gt;
&lt;img src="http://example.com/graph1" alt="(The trend is for drinks to get sweeter)" title="Figure One" <strong>longdesc="#graph1"</strong>&gt;

&lt;!-- The description is a page on its own --&gt;
&lt;img src="figure1" alt="figure 1" <strong>longdesc="http://example.com/desc1"</strong>&gt;

&lt;!-- The description is one of several within an external page --&gt;
&lt;img src="ExampleImage" alt="example" <strong>longdesc="http://example.com/descs#item3"</strong>&gt;

&lt;!-- The description is included in a data: URI --&gt;
&lt;!-- NOTE: Plain text is only suitable for simple descriptions --&gt;
&lt;img src="logo" alt="W3C" <strong>longdesc="data:text/plain;charset=utf-8;,A%20blue%20capital%20letter%20W%2C%20with%20kerning%20
so%20it%20touches%20a%20blue%203%2C%20followed%20by%20the%20black%20shadow%20of%20a%20white%20capital%20letter%20C
%20all%20on%20a%20white%20background"</strong>&gt;</code></pre>
    </section>
    <section id="conformance">
      <h2><span class="secno">1. </span>Conformance</h2>
      <p> As well as sections marked as non-normative, all authoring guidelines,
        diagrams, examples, and notes in this specification are non-normative.
        Everything else in this specification is normative. </p>
      <p> The key words <em title="must" class="rfc2119">must</em>, <em title="should"
          class="rfc2119">should</em>,
        <em title="should not" class="rfc2119"></em><em title="recommended" class="rfc2119"></em>and
         <em title="may" class="rfc2119">may</em><em title="optional" class="rfc2119"></em>
        in this specification are to be interpreted as described in [<cite><a class="bibref"
            href="#bib-RFC2119">RFC2119</a></cite>].</p>
      <section id="dependencies">
        <h3><span class="secno">1.1 </span>Dependencies</h3>
        <p>The IDL fragments in this specification <em title="must" class="rfc2119">must</em>
          be interpreted as required for conforming IDL fragments, as described
          in the Web IDL specification. [<cite><a class="bibref" href="#bib-WEBIDL">WEBIDL</a></cite>]</p>
        <p>The terms "<a href="http://dev.w3.org/html5/spec/urls.html#valid-url-potentially-surrounded-by-spaces">valid
-            URL potentially surrounded by spaces</a>", "<a href="http://dev.w3.org/html5/spec/links.html#hyperlink">hyperlink"</a>,
          and "<a href="http://www.w3.org/TR/html5/infrastructure.html#reflect">reflect</a>",
+            non-emptyURL potentially surrounded by spaces</a>", "<a href="http://dev.w3.org/html5/spec/links.html#hyperlink">hyperlink"</a>,
          and "<a href="http://www.w3.org/TR/html5/infrastructure.html#reflect">reflect</a>",
           used in this specification are defined in<cite><a class="bibref" href="#bib-DOM4"></a></cite>
          [<cite><a class="bibref" href="#bib-HTML5">HTML5</a></cite>]<cite><a class="bibref"
              href="#bib-XML10"></a></cite>.
        </p>
      </section>
    </section>
    <section id="UCnR">
      <h2><span class="secno">2. </span>Use Cases and Requirements</h2>
      <p>There are many ways users can successfully interact with content even
        if they cannot see, or see well, images included in that content. The <code>alt</code>
        attribute is designed to ensure that for everyday work, a user has
        enough information to replace an image, and often this is more helpful
        than a detailed description of every image. The <code>longdesc</code>
        attribute is designed to complement this functionality, to meet the
        following use cases.<br>
      </p>
      <h3>Use Cases</h3>
      <dl>
        <dt>Describing a well-known image</dt>
      </dl>
      <dl>
        <dd>There are many well-known images which are widely reproduced - Mona
          Lisa, Washington crossing some river, the Da Vinci picture of the guy
          in a square and a circle, Gerníka, Cubbin's "Lost", Leunig's "ramming
          the shears". While different people know the images by different
          titles, and some images have no widely known titles, visually
          recognising the image is often an important part of framing a
          discussion. Where the image itself cannot be seen, a description can
          often be used to offer the same recognition.</dd>
        <dd><em>Requires</em>: Discoverability</dd>
        <dd><em>Helped by</em>: Inline, Reuse</dd>
        <dt>Describing a complex diagram</dt>
        <dd>In many environments diagrams are used to explain a concept or
          transmit information efficiently. Where a user has a reduced ability
          to see the image (poor contrast vision and other conditions are as
          relevant here as a complete lack of vision), a description can enable
          that user to understand the information being presented</dd>
        <dd><em>Requires</em>: Structured Markup, Inline, Reuse, Simple Return</dd>
        <dt>Teaching Accessible Development</dt>
        <dd>In many environments producing content that is accessible to users
          regardless of disability is a legal and/or market requirement. </dd>
        <dd><em>Requires</em>: Maintenance, Backwards compatibility</dd>
        <dt>A self-describing artistic work</dt>
        <dd>For many pages the visual design of the page is an important part of
          the message it conveys to a fully-sighted user, but the author would
          like to convey as much of that design as possible to a user with low
          or no vision available. </dd>
        <dd><em>Requires</em>: No visual encumbrance, Structured Markup</dd>
        <dd><em>Helped by</em>: Reuse, Simple Return</dd>
        <dt>Referring to an existing description</dt>
        <dd>Many well-known images are already described by other sources. The
          copyright on those sources may not be compatible with re-publishing
          the description, but there is little value in making a new one. </dd>
        <dd><em>Requires</em>: Reuse, Structured Markup, Simple Return, Optional
          Consumption<br>
        </dd>
        <dd><em>Helped by</em>: No Visual Encumbrance</dd>
        <dt>Linking to a description included within a page</dt>
        <dd>In some cases an image already has a description included within a
          page. Making the linkage explicit can provide further clarity for a
          user navigating the page who is not able to interpret the default
          layout, e.g. because they have forced a re-layout of the page
          elements, or because they do not see the default visual relationship
          between the element and its description.</dd>
        <dd>This practice also enables description to be provided for all users.</dd>
        <dd>By keeping the association clear the content maintainer can more
          easily check that the description and link are actually correct.</dd>
        <dd><em>Requires</em>: Inline, Simple Return, Structured Markup,
          Optional Consumption<br>
        </dd>
      </dl>
      <h3>Requirements </h3>
      <dl>
        <dt>No visual encumbrance</dt>
        <dd>It <em class="rfc2119">must</em> be possible to provide a
          description for an image without any visual encumbrance on the page.</dd>
        <dt>Structured Markup</dt>
        <dd>It <em class="rfc2119">must</em> be possible to include rich markup
          (e.g. HTML5) in the description of the image.</dd>
        <dt>Inline</dt>
        <dd>It <em class="rfc2119">must</em> be possible to associate a
          description in the body of a page with an image in that page. </dd>
        <dt>Reuse</dt>
        <dd>It <em class="rfc2119">must</em> be possible to re-use a single
          description for multiple occurrences of an image.</dd>
        <dt>Maintenance</dt>
        <dd>It <em class="rfc2119">must</em> be simple to maintain a library of
          images and descriptions for dynamic assembly, or dis-assembly and
          re-assembly, of content. </dd>
        <dt>Discoverability </dt>
        <dd class="rfc2119">It <em class="rfc2119">must</em> be simple for a
          user agent to automatically discover a description provided for a
          given image.</dd>
        <dd>A user <em class="rfc2119">should</em> be able to determine that
          there is a description available for a given image.<br>
        </dd>
        <dt>Optional consumption</dt>
        <dd>A user <em class="rfc2119">must</em> be able to choose not to read
          the long description of a given image.</dd>
        <dt>Simple return</dt>
        <dd>A user <em class="rfc2119">must</em> be able to return from the
          description to the image. </dd>
        <dt>Backwards Compatibility </dt>
        <dd>It <em class="rfc2119">should</em> be possible to use existing
          tools and techniques to associate an image with its description. </dd>
      </dl>
    </section>
    <section id="longdesc">
      <h2><span class="secno"></span>3. The <code>longdesc</code> attribute</h2>
      <section id="parsing">
        <p class="rfc2119">Zero or one <code>longdesc</code> attributes <em class="rfc2119">may</em>
          be added to any <code>img</code> element.</p>
        <p>The <code>longdesc</code> attribute <em class="rfc2119">must</em>
          contain a <a href="http://dev.w3.org/html5/spec/urls.html#valid-url-potentially-surrounded-by-spaces">valid
-            URL potentially surrounded by spaces</a>. The URL is a <a href="http://dev.w3.org/html5/spec/links.html#hyperlink">hyperlink</a>
          to a description of the image that its parent <code>img</code>
          element represents.</p>
        <p>Authors <em class="rfc2119">should</em> put descriptions within an
          element which is the target of a fragment link (e.g. <code>longdesc="example.html#description"</code>)
          if a description is only part of the target document. </p>
        <p>User agents <em class="rfc2119">should</em> make the link available
          to all users through the regular user interface. <br>
        </p>
        <p>User agents <em class="rfc2119">should</em> expose the link to
          relevant APIs, especially accessibility-oriented APIs.<br>
        </p>
        <p>User agents <em class="rfc2119">should</em> enable users to discover
          when images in a page contain a long description.</p>
        <p>If a <code>longdesc</code> attribute has invalid content, user
          agents <em class="rfc2119">may</em> make that content available to
          the user. This is because a common authoring error is to include the
          text of a description, instead of the URL of a description, as the
          value of the attribute.</p>
        <div class="note">
          <p><em>This section is informative</em></p>
          <p>Best practices for full descriptions of images are beyond the scope
            of this document, but there are many resources available.</p>
          <p>Best practices for implementation are beyond the scope of this
            document. Since some images with long description are also normal
            link content, it is important for implementations to ensure that
            users can activate both behaviours. Likewise, it is generally
            helpful when the behaviour for finding, reading, and returning from
            a long description to the image described is a consistent
            experience. </p>
        </div>
      </section>
      <section id="extensions-to-the-element-interface">
        <h3><span class="secno">3.1 </span>Extension to the <code><a title="element"
              class="externalDFN"
              data-spec="HTML5">HTMLImageElement</a></code>
          interface</h3>
        <br>
        <pre class="idl"><span class="idlInterface" id="idl-def-Element">partial interface <span
class="idlInterfaceID">HTMLImageElement</span> {
<span class="idlAttribute">             attribute <span class="idlAttrType"><a>DOMString</a></span> <span
class="idlAttrName"><a
href="#widl-Element-longdesc">longdesc</a></span>;</span>
};</span></pre>
        <section id="attributes">
          <h4><span class="secno">3.1.1 </span>Attribute</h4>
          <dl class="attributes">
            <dt id="widl-Element-longdesc"><code>longdesc</code> of type <span
                class="idlAttrType"><a>DOMString</a></span></dt>
            <dd>
              <p class="rfc2119">The <dfn id="dfn-dom-element-longdesc"><code>longdesc</code></dfn>
                IDL attribute <em class="rfc2119">must</em> be a valid
                non-empty URL potentially surrounded by spaces. It represents a
                hyperlink to a detailed description of the image its parent <code><a
                    title="element"
                    class="externalDFN"
                    data-spec="DOM4">HTMLImageElement</a></code>
                represents.</p>
            </dd>
            <dd>
              <p>The <code>longdesc</code> IDL attribute <em class="rfc2119">must</em>
                <a href="http://www.w3.org/TR/html5/infrastructure.html#reflect">reflect</a>
                the HTML content attribute <code>longdesc</code>.<br>
              </p>
            </dd>
            <dd>
              <dl class="domintro">
                <dt><var>image</var>.<code title="dom-Element-innerHTML">longdesc</code>
                  [ = <var>value</var> ] </dt>
                <dd>
                  <p>Returns a DOMString that represents the attribute's
                    contents. </p>
                  <p>Can be set, to replace the contents of the attribute with
                    the given string.</p>
                </dd>
              </dl>
            </dd>
          </dl>
          <pre class="example"><code>//Make the first internal longdesc reference absolute<br><br>var baseURL = document.location.origin + document.location.pathname<br>var someImage = document.querySelector('img[longdesc^=#]');<br><br>someImage.longdesc = baseURL + someImage.longdesc;</code></pre>
          <pre class="example"><code>//Open new windows for each longdesc found<br><br>var describedImages = document.querySelectorAll('img[longdesc]');<br><br>for (i in describedImages) {<br>  if (i.<strong>longdesc</strong>) //there are a couple of extra things matched <br>    window.open(i.<strong>longdesc</strong>);<br>}</code></pre>
        </section>
      </section>
      <section class="appendix" id="acknowledgements">
        <h2><span class="secno">A. </span>Acknowledgements</h2>
        <p>Thanks to the HTML Working group of the late 1990s for the original
          specification of longdesc, to those who have implemented it in various
          kinds of software, and to many many people involved with the
          development of HTML5 (including but not limited to those who discussed
          "ISSUE-30" in the HTML Working Group, the Protocols and Formats
          Working Group, the W3C Advisory Board, and around countless dinner
          tables, coffee breaks, and elsewhere) for the ideas, discussions and
          contributions that led to this specification. With the exception of
          Laura Carlson, who did far more very valuable work than it took me to
          produce this specification, I haven't named them: the list might be
          larger than the content of the specification. </p>
      </section>
      <section class="appendix" id="references">
        <h2><span class="secno">B. </span>References</h2>
        <section id="normative-references">
          <h3><span class="secno">B.1 </span>Normative references</h3>
          <dl class="bibliography">
            <dt id="bib-HTML5">[HTML5]</dt>
            <dd>Robin Berjon et. al., eds. <a href="http://www.w3.org/TR/html5"><cite>HTML5.</cite></a>
              W3C Working Draft. (Work in progress.) URL: <a href="http://www.w3.org/TR/html5">http://www.w3.org/TR/html5</a>
            </dd>
            <dt id="bib-RFC2119">[RFC2119]</dt>
            <dd>S. Bradner. <a href="http://www.ietf.org/rfc/rfc2119.txt"><cite>Key
-                  words for use in RFCs to Indicate Requirement Levels.</cite></a>
              March 1997. Internet RFC 2119. URL: <a href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a>
            </dd>
            <dt id="bib-WEBIDL">[WEBIDL]</dt>
            <dd>Cameron McCormack. <a href="http://www.w3.org/TR/2011/WD-WebIDL-20110927/"><cite>Web
-                  IDL.</cite></a> 27 September 2011. W3C Working Draft. (Work in
              progress.) URL: <a href="http://www.w3.org/TR/2011/WD-WebIDL-20110927/">http://www.w3.org/TR/2011/WD-WebIDL-20110927/</a>
            </dd>
          </dl>
        </section>
      </section>
    </section>
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+            non-empty URL potentially surrounded by spaces</a>. The URL is a <a
            href="http://dev.w3.org/html5/spec/links.html#hyperlink">hyperlink</a>
          to a description of the image that its parent <code>img</code>
          element represents. The linked description should be a broadly
          accessible format. (for example, a link to another version of the
          image is not a useful description).</p>
        <p>Authors <em class="rfc2119">should</em> put descriptions within an
          element which is the target of a fragment link (e.g. <code>longdesc="example.html#description"</code>)
          if a description is only part of the target document. </p>
        <div class="note">
          <p><em>This section is informative</em></p>
          <p>Best practices for full descriptions of images are beyond the scope
            of this document, but there are many resources available.</p>
          <p>This document does not define accessibility. For further guidance
            on making the description an accessible document, authors should
            consult [<a href="#bib-WCAG">WCAG</a>]</p>
        </div>
        <p>User agents <em class="rfc2119">should</em> make the link available
          to all users through the regular user interface. <br>
        </p>
        <p>User agents <em class="rfc2119">should</em> expose the link to
          relevant APIs, especially accessibility-oriented APIs.<br>
        </p>
        <p>User agents <em class="rfc2119">should</em> enable users to discover
          when images in a page contain a long description.</p>
        <p>If a <code>longdesc</code> attribute has invalid content, user
          agents <em class="rfc2119">may</em> make that content available to
          the user. This is because a common authoring error is to include the
          text of a description, instead of the URL of a description, as the
          value of the attribute.</p>
        <div class="note">
          <p><em>This section is informative</em></p>
          <p>Complete documentation of best practices for implementation is
            beyond the scope of this document. However the following notes are
            offered to help repair a common authoring mistake and avoid a common
            user agent mistake.</p>
          <p>One of the most common mistakes authors make that is easily
            repaired by user agents is to use a description, instead of a URL
            that links to a description. This means there is often plain text
            description in the content of an invalid longdesc attribute.
            Converting such attributes to data URLs is a simple repair strategy
            that can help recover from cases where authors have made this
            mistake. </p>
          <p>Since some images with long description are also normal link
            content, it is important for implementations to ensure that users
            can activate both behaviours. Likewise, it is generally helpful when
            the behaviour for finding, reading, and returning from a long
            description to the image described is a consistent experience. </p>
        </div>
      </section>
      <section id="extensions-to-the-element-interface">
        <h3><span class="secno">3.1 </span>Extension to the <code><a title="element"
              class="externalDFN"
              data-spec="HTML5">HTMLImageElement</a></code>
          interface</h3>
        <pre class="idl"><span class="idlInterface" id="idl-def-Element">partial interface <span
class="idlInterfaceID">HTMLImageElement</span> {
<span class="idlAttribute">             attribute <span class="idlAttrType"><a>DOMString</a></span> <span
class="idlAttrName"><a
href="#widl-Element-longdesc">longdesc</a></span>;</span>
};</span></pre>
        <section id="attributes">
          <h4><span class="secno">3.1.1 </span>Attribute</h4>
          <dl class="attributes">
            <dt id="widl-Element-longdesc"><code>longdesc</code> of type <span
                class="idlAttrType"><a>DOMString</a></span></dt>
            <dd>
              <p class="rfc2119">The <dfn id="dfn-dom-element-longdesc"><code>longdesc</code></dfn>
                IDL attribute <em class="rfc2119">must</em> be a valid
                non-empty URL potentially surrounded by spaces. It represents a
                hyperlink to a detailed description of the image its parent <code><a
                    title="element"
                    class="externalDFN"
                    data-spec="DOM4">HTMLImageElement</a></code>
                represents.</p>
            </dd>
            <dd>
              <p>The <code>longdesc</code> IDL attribute <em class="rfc2119">must</em>
                <a href="http://www.w3.org/TR/html5/infrastructure.html#reflect">reflect</a>
                the HTML content attribute <code>longdesc</code>.<br>
              </p>
            </dd>
            <dd>
              <dl class="domintro">
                <dt><var>image</var>.<code title="dom-Element-innerHTML">longdesc</code>
                  [ = <var>value</var> ] </dt>
                <dd>
                  <p>Returns a DOMString that represents the attribute's
                    contents. </p>
                  <p>Can be set, to replace the contents of the attribute with
                    the given string.</p>
                </dd>
              </dl>
            </dd>
          </dl>
          <pre class="example"><code>//Make the first internal longdesc reference absolute

var baseURL = document.location.origin + document.location.pathname
var someImage = document.querySelector('img[longdesc^=#]');

someImage.longdesc = baseURL + someImage.longdesc;</code></pre>
          <pre class="example"><code>//Open new windows for each longdesc found

var describedImages = document.querySelectorAll('img[longdesc]');

for (i in describedImages) {
  if (i.<strong>longdesc</strong>)
    window.open(i.<strong>longdesc</strong>);
}</code></pre>
          <pre class="example"><code>//Tries to repair errors where the longdesc isn't a URI

var describedImages = document.querySelectorAll('img[longdesc]');

for (i in describedImages) {
  if (i.longdesc &amp;&amp; !(validURL(i.longdesc)) { //assumes some URL validating function
    var theData = encodeURIComponent(i.longdesc);
    i.longdesc = "data:text/plain;charset=";
    i.longdesc += document.charset;
    i.longdesc += theData;
  }
}</code></pre>
        </section>
      </section>
      <section class="appendix" id="acknowledgements">
        <h2><span class="secno">A. </span>Acknowledgements</h2>
        <p>Thanks to the HTML Working group of the late 1990s for the original
          specification of longdesc, to those who have implemented it in various
          kinds of software, and to many many people involved with the
          development of HTML5 (including but not limited to those who discussed
          "ISSUE-30" in the HTML Working Group, the Protocols and Formats
          Working Group, the W3C Advisory Board, and around countless dinner
          tables, coffee breaks, and elsewhere) for the ideas, discussions and
          contributions that led to this specification. With the exception of
          Laura Carlson, who did far more very valuable work than it took me to
          produce this specification, I haven't named them: the list might be
          larger than the content of the specification.</p>
        <p>For specific comments and suggestions that led to improvements over
          successive drafts of this specification, thanks to the W3C's HTML
          Accessibility Task Force, and to Robin Berjon, James Craig, John
          Foliot, Geoff Freed, Anne van Kesteren, David McDonald, Leif Halvard
          Silli, Mathew Turvey. Any errors are despite, not because of, their
          efforts.</p>
      </section>
      <section class="appendix" id="references">
        <h2><span class="secno">B. </span>References</h2>
        <section id="normative-references">
          <h3><span class="secno">B.1 </span>Normative references</h3>
          <dl class="bibliography">
            <dt id="bib-HTML5">[HTML5]</dt>
            <dd>Robin Berjon et. al., eds. <a href="http://www.w3.org/TR/html5"><cite>HTML5.</cite></a>
              W3C Working Draft. (Work in progress.) URL: <a href="http://www.w3.org/TR/html5">http://www.w3.org/TR/html5</a>
            </dd>
            <dt id="bib-RFC2119">[RFC2119]</dt>
            <dd>S. Bradner. <a href="http://www.ietf.org/rfc/rfc2119.txt"><cite>Key
+                  words for use in RFCs to Indicate Requirement Levels.</cite></a>
              March 1997. Internet RFC 2119. URL: <a href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a></dd>
            <dt id="bib-WEBIDL">[WEBIDL]</dt>
            <dd>Cameron McCormack. <a href="http://www.w3.org/TR/2011/WD-WebIDL-20110927/"><cite>Web
+                  IDL.</cite></a> 27 September 2011. W3C Working Draft. (Work in
              progress.) URL: <a href="http://www.w3.org/TR/2011/WD-WebIDL-20110927/">http://www.w3.org/TR/2011/WD-WebIDL-20110927/</a></dd>
          </dl>
        </section>
        <section id="informative-references">
          <h3><span class="secno">B.2 </span>Informative references</h3>
          <dl class="bibliography">
            <dt id="bib-WCAG2">[WCAG]</dt>
            <dd>Ben Caldwell et. al., eds. <a href="http://www.w3.org/TR/2008/REC-WCAG20-20081211/"><cite>Web
                  Content Accessibility Guidelines 2.0</cite></a>, December
              2008. W3C Recommendation. URL: <a href="http://www.w3.org/TR/2008/REC-WCAG20-20081211/">http://www.w3.org/TR/2008/REC-WCAG20-20081211/</a></dd>
          </dl>
          <p></p>
          <dl class="bibliography">
          </dl>
        </section>
      </section>
    </section>
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