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<div class="head"><p><a href="http://www.w3.org/"><img src="http://www.w3.org/Icons/w3c_home" alt="W3C" height="48" width="72"></a></p>
<h1>XML Linking Language (XLink) Version 1.0</h1>
<h2>W3C Recommendation 27 June 2001</h2>
<div id="xlink11_notice" style="border: solid black 1px;
 padding: 0.5em; background: #FFB;">
<p style="margin-top: 0; font-weight: bold;">New Version
Available: XLink 1.1 <span style="padding-left: 2em;"></span>
(Document Status Update, 14 September 2010)</p>
<p style="margin-bottom: 0;">The XML Core Working Group has produced
a W3C Recommendation for a new version of XLink which adds
features to this 2001 version while remaining compatible.
Please see <a href="http://www.w3.org/TR/xlink11/">XLink 1.1</a>
for the latest version.</p>
</div>
<dl><dt>This version:</dt><dd><a href="http://www.w3.org/TR/2001/REC-xlink-20010627/">http://www.w3.org/TR/2000/REC-xlink-20010627/</a>(available in <a href="http://www.w3.org/TR/2001/REC-xlink-20010627/Overview.html">HTML</a>, <a href="http://www.w3.org/TR/2001/REC-xlink-20010627/Overview.xml">XML</a>)</dd><dt>Latest version:</dt><dd><a href="http://www.w3.org/TR/xlink/">http://www.w3.org/TR/xlink/</a></dd><dt>Previous versions:</dt><dd><a href="http://www.w3.org/TR/2000/PR-xlink-20001220/">http://www.w3.org/TR/2000/PR-xlink-20001220/</a></dd><dt>Editors:</dt>
<dd>Steve DeRose, Brown University Scholarly Technology
Group</dd>
<dd>Eve Maler, Sun Microsystems</dd>
<dd>David Orchard, Jamcracker</dd>
</dl><p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#Copyright">Copyright</a>&nbsp;©&nbsp;2001&nbsp;<a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a href="http://www.lcs.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href="http://www.inria.fr/"><abbr lang="fr" title="Institut National de Recherche en Informatique et Automatique">INRIA</abbr></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#W3C_Trademarks">trademark</a>, <a href="http://www.w3.org/Consortium/Legal/copyright-documents-19990405">document use</a>, and <a href="http://www.w3.org/Consortium/Legal/copyright-software-19980720">software licensing</a> rules apply.</p></div><hr><div>
<h2><a name="abstract">Abstract</a></h2>
<p>This specification defines the XML Linking Language (XLink), which allows
elements to be inserted into XML documents in order to create and describe
links between resources. It uses XML syntax to create structures that can
describe links similar to the simple unidirectional hyperlinks of today's
HTML, as well as more sophisticated links.</p>
</div><div>
<h2><a name="status">Status of this Document</a></h2>

<p>This document has been reviewed by W3C Members and other
   interested parties and has been endorsed by the Director
   as a W3C Recommendation. It is a stable document and may
   be used as reference material or cited as a normative
   reference from another document.
   W3C's role in making the Recommendation is to draw attention to the
   specification and to promote its widespread deployment. This
   enhances the functionality and interoperability of the Web.
</p><p>For information about the XPointer language that <a title="Must, May, etc." href="#dt-must">may</a>
be used with XLink, see <a href="#xptr">[XPTR]</a>.</p>
<p>This document has been produced by the W3C XML Linking Working
        Group as part of the XML Activity in the W3C Architecture
        Domain.  For background on this work, please see the <a href="http://www.w3.org/XML/Activity">XML
Activity Statement</a>.</p>
<p>Please report possible errors in this
document to the public email list <a href="mailto:www-xml-linking-comments@w3.org">www-xml-linking-comments@w3.org</a> (archive
   at
   <a href="http://lists.w3.org/Archives/Public/www-xml-linking-comments/">http://lists.w3.org/Archives/Public/www-xml-linking-comments/</a>). Any
   confirmed errors will be documented in an list of errata available
   at <a href="http://www.w3.org/2001/06/xlink-errata">http://www.w3.org/2001/06/xlink-errata</a>.</p>
<p>The English version of this specification is the only normative
   version. Information about translations of this document is available
   at <a href="http://www.w3.org/2001/06/xlink-translations">http://www.w3.org/2001/06/xlink-translations</a>.</p>
<p>See <a href="#xldp">[XLDP]</a> for additional background on the design principles
informing XLink, and <a href="#xlreq">[XLREQ]</a> for the normative XLink requirements
that this document attempts to satisfy. XLink does not support all HTML linking
constructs as they stand; see <a href="#xlink-naming">[XLinkNaming]</a> for a discussion
of this situation.</p>
<p>A list of current W3C Recommendations and other technical documents can
be found at <a href="http://www.w3.org/TR/">http://www.w3.org/TR/</a>.</p>
</div>
<div class="toc">
<h2><a name="contents">Table of Contents</a></h2><p class="toc">1 <a href="#intro">Introduction</a><br>&nbsp;&nbsp;&nbsp;&nbsp;1.1 <a href="#origin-goals">Origin and Goals</a><br>2 <a href="#N781">XLink Concepts</a><br>&nbsp;&nbsp;&nbsp;&nbsp;2.1 <a href="#N789">Links and Resources</a><br>&nbsp;&nbsp;&nbsp;&nbsp;2.2 <a href="#N854">Arcs, Traversal, and Behavior</a><br>&nbsp;&nbsp;&nbsp;&nbsp;2.3 <a href="#N899">Resources in Relation to the Physical Location of a Linking Element</a><br>3 <a href="#conformance">XLink Processing and Conformance</a><br>&nbsp;&nbsp;&nbsp;&nbsp;3.1 <a href="#N1022">Processing Dependencies</a><br>&nbsp;&nbsp;&nbsp;&nbsp;3.2 <a href="#markup-reqs">Markup Conformance</a><br>&nbsp;&nbsp;&nbsp;&nbsp;3.3 <a href="#app-reqs">Application Conformance</a><br>4 <a href="#att-method">XLink Markup Design</a><br>&nbsp;&nbsp;&nbsp;&nbsp;4.1 <a href="#N1238">XLink Attribute Usage Patterns</a><br>&nbsp;&nbsp;&nbsp;&nbsp;4.2 <a href="#N1599">XLink Element Type Relationships</a><br>&nbsp;&nbsp;&nbsp;&nbsp;4.3 <a href="#defaulting">Attribute Value Defaulting</a><br>&nbsp;&nbsp;&nbsp;&nbsp;4.4 <a href="#integrating">Integrating XLink Usage with Other Markup</a><br>&nbsp;&nbsp;&nbsp;&nbsp;4.5 <a href="#legacy">Using XLink with Legacy Markup</a><br>5 <a href="#linking-elements">XLink Elements and Attributes</a><br>&nbsp;&nbsp;&nbsp;&nbsp;5.1 <a href="#extended-link">Extended Links (extended-Type Element)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.1.1 <a href="#local-resource">Local Resources for an Extended Link  (resource-Type Element)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.1.2 <a href="#remote-resource">Remote Resources for an Extended Link (locator-Type Element)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.1.3 <a href="#xlink-arcs">Traversal Rules for an Extended Link  (arc-Type Element)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.1.4 <a href="#title-element">Titles for Extended Links, Locators, and Arcs (title-Type Element)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.1.5 <a href="#xlg">Locating Linkbases (Special Arc Role)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;5.2 <a href="#simple-links">Simple Links (simple-Type Element)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;5.3 <a href="#link-types">XLink Element Type Attribute (type)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;5.4 <a href="#link-locators">Locator Attribute (href)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;5.5 <a href="#link-semantics">Semantic Attributes (role, arcrole, and title)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;5.6 <a href="#link-behaviors">Behavior Attributes (show and actuate) </a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.6.1 <a href="#show-att">show Attribute</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.6.2 <a href="#actuate-att">actuate Attribute</a><br>&nbsp;&nbsp;&nbsp;&nbsp;5.7 <a href="#traversal-atts">Traversal Attributes (label, from, and to)</a><br></p>
<h3>Appendices</h3><p class="toc">A <a href="#references">References</a><br>&nbsp;&nbsp;&nbsp;&nbsp;A.1 <a href="#N3954">Normative References</a><br>&nbsp;&nbsp;&nbsp;&nbsp;A.2 <a href="#N4107">Non-Normative References</a><br>B <a href="#sample-dtd-appx">Sample DTD</a> (Non-Normative)<br>C <a href="#acknowledgements">Working Group Members and Acknowledgments</a> (Non-Normative)<br></p></div><hr><div class="body">
<div class="div1">

<h2><a name="intro"></a>1 Introduction</h2>
<p>This specification defines the XML Linking Language (XLink), which allows
elements to be inserted into XML documents in order to create and describe <a title="Link" href="#dt-link">links</a> between resources.</p>
<p>XLink provides a framework for creating both basic unidirectional links
and more complex linking structures. It allows XML documents to:</p>
<ul>
<li><p>Assert linking relationships among more than two resources</p></li>
<li><p>Associate metadata with a link</p></li>
<li><p>Express links that reside in a location separate from the linked
resources</p></li>
</ul>
<p>An important application of XLink is in hypermedia systems that have <a title="Hyperlink" href="#dt-hyperlink">hyperlinks</a>. A simple case of a hyperlink is an
HTML <code>A</code> element, which has these characteristics:</p>
<ul>
<li><p>The hyperlink uses URIs as its locator technology.</p></li>
<li><p>The hyperlink is expressed at one of its two ends.</p></li>
<li><p>The hyperlink identifies the other end (although a server may have
great freedom in finding or dynamically creating that destination).</p></li>
<li><p>Users can initiate traversal only from the end where the hyperlink
is expressed to the other end.</p></li>
<li><p>The hyperlink's effect on windows, frames, go-back lists, style sheets
in use, and so on is determined by user agents, not by the hyperlink itself.
For example, traversal of <code>A</code> links normally replaces the current
view, perhaps with a user option to open a new window.</p></li>
</ul>
<p>This set of characteristics is powerful, but the model that underlies them
limits the range of possible hyperlink functionality. The model defined in
this specification shares with HTML the use of URI technology, but goes beyond
HTML in offering features, previously available only in dedicated hypermedia
systems, that make hyperlinking more scalable and flexible. Along with providing
linking data structures, XLink provides a minimal link behavior model; higher-level
applications layered on XLink will often specify alternate or more sophisticated
rendering and processing treatments.</p>
<p>Integrated treatment of specialized links used in other technical domains,
such as foreign keys in relational databases and reference values in programming
languages, is outside the scope of this specification.</p>
<div class="div2">

<h3><a name="origin-goals"></a>1.1 Origin and Goals</h3>
<p>The design of XLink has been informed by knowledge of established hypermedia
systems and standards. The following standards have been especially influential:</p>
<ul>
<li><p><em>HTML</em> <a href="#html">[HTML]</a>: Defines several element types
that represent links.</p></li>
<li><p><em>HyTime</em> <a href="#iso10744">[ISO/IEC 10744]</a>: Defines inline and
inbound and third-party link structures and some semantic features, including
traversal control and presentation of objects.</p></li>
<li><p><em>Text Encoding Initiative Guidelines</em> <a href="#tei">[TEI]</a>:
Provides structures for creating links, aggregate objects, and link collections.</p>
</li>
</ul>
<p>Many other linking systems have also informed the design of XLink, especially <a href="#dexter">[Dexter]</a>, <a href="#fress">[FRESS]</a>, <a href="#ohs">[OHS]</a>, <a href="#microcosm">[MicroCosm]</a>,
and <a href="#intermedia">[Intermedia]</a>.</p>
<p>See the XLink Requirements Document <a href="#xlreq">[XLREQ]</a> for a thorough
explanation of requirements for the design of XLink.</p>
</div>
</div>
<div class="div1">

<h2><a name="N781"></a>2 XLink Concepts</h2>
<p>This section describes the terms and concepts that are essential to understanding
XLink, without discussing the syntax used to create XLink constructs. A few
additional terms are introduced in later parts of this specification.</p>
<div class="div2">

<h3><a name="N789"></a>2.1 Links and Resources</h3>
<p>[<a name="dt-link" title="Link">Definition</a>: An XLink <b>link</b> is an explicit
relationship between resources or portions of resources.] [<a name="dt-linkel" title="Linking element">Definition</a>: It is made explicit by an XLink <b>linking
element</b>, which is an XLink-conforming XML element that asserts the
existence of a link.] There are six XLink elements; only two of them
are considered linking elements. The others provide various pieces of information
that describe the characteristics of a link. (The term "link"
as used in this specification refers only to an XLink link, though nothing
prevents non-XLink constructs from serving as links.)</p>
<p>The notion of resources is universal to the World Wide Web. [<a name="dt-resource" title="Resource">Definition</a>: As discussed in <a href="#rfc2396">[IETF RFC 2396]</a>, a <b>resource</b>
is any addressable unit of information or service.] Examples include
files, images, documents, programs, and query results. The means used for
addressing a resource is a URI (Uniform Resource Identifier) reference (described
more in <a href="#link-locators"><b>5.4 Locator Attribute (href)</b></a>). It is possible to address a portion
of a resource. For example, if the whole resource is an XML document, a useful
portion of that resource might be a particular element inside the document.
Following a link to it might result, for example, in highlighting that element
or scrolling to that point in the document.</p>
<p>[<a name="dt-particip-resource" title="Participating resource">Definition</a>: When a
link associates a set of resources, those resources are said to <b>participate</b>
in the link.] Even though XLink links must appear in XML documents,
they are able to associate all kinds of resources, not just XML-encoded ones.</p>
<p>One of the common uses of XLink is to create hyperlinks. [<a name="dt-hyperlink" title="Hyperlink">Definition</a>: A <b>hyperlink</b> is a link that is intended primarily
for presentation to a human user.] Nothing in XLink's design, however,
prevents it from being used with links that are intended solely for consumption
by computers.</p>
</div>
<div class="div2">

<h3><a name="N854"></a>2.2 Arcs, Traversal, and Behavior</h3>
<p>[<a name="dt-traversal" title="Traversal">Definition</a>: Using or following a link for
any purpose is called <b>traversal</b>.] Even though some kinds
of link can associate arbitrary numbers of resources, traversal always involves
a pair of resources (or portions of them); [<a name="dt-starting-resource" title="Starting resource">Definition</a>: the source from which traversal is begun is the <b>starting
resource</b>] and [<a name="dt-ending-resource" title="Ending resource">Definition</a>: the
destination is the <b>ending resource</b>]. Note that the term "resource"
used in this fashion may at times apply to a resource portion, not a whole
resource.</p>
<p>[<a name="dt-arc" title="Arc">Definition</a>: Information about how to traverse a pair
of resources, including the direction of traversal and possibly application
behavior information as well, is called an <b>arc</b>]. If
two arcs in a link specify the same pair of resources, but they switch places
as starting and ending resources, then the link is multidirectional, which
is not the same as merely "going back" after traversing a link.</p>
</div>
<div class="div2">

<h3><a name="N899"></a>2.3 Resources in Relation to the Physical Location of a Linking Element</h3>
<p>[<a name="dt-local-resource" title="Local resource">Definition</a>: A <b>local resource</b>
is an XML element that participates in a link by virtue of having as its parent,
or being itself, a linking element]. [<a name="dt-remote-resource" title="Remote resource">Definition</a>: Any resource or resource portion that participates
in a link by virtue of being addressed with a URI reference is considered
a <b>remote resource</b>, even if it is in the same XML document as
the link, or even inside the same linking element.] Put another way,
a local resource is specified "by value," and a remote resource
is specified "by reference."</p>
<p>[<a name="dt-outbound" title="Outbound">Definition</a>: An arc that has a local starting
resource and a remote ending resource goes <b>outbound</b>, that is,
away from the linking element.] (Examples of links with such an arc
are the HTML <code>A</code> element, HyTime "clinks," and Text Encoding
Initiative <code>XREF</code> elements.) [<a name="dt-inbound" title="Inbound">Definition</a>: If
an arc's ending resource is local but its starting resource is remote, then
the arc goes <b>inbound</b>.] [<a name="dt-third-party" title="Third-party">Definition</a>: If neither the starting resource nor the ending resource
is local, then the arc is a <b>third-party</b> arc.] Though
it is not required, any one link typically specifies only one kind of arc
throughout, and thus might be referred to as an inbound, outbound, or third-party
link.</p>
<p>To create a link that emanates from a resource to which you do not have
(or choose not to exercise) write access, or from a resource that offers no
way to embed linking constructs, it is necessary to use an inbound or third-party
arc. When such arcs are used, the requirements for discovery of the link are
greater than for outbound arcs. [<a name="dt-linkbase" title="Linkbase">Definition</a>: Documents
containing collections of inbound and third-party links are called link databases,
or <b>linkbases</b>.]</p>
</div>
</div>
<div class="div1">

<h2><a name="conformance"></a>3 XLink Processing and Conformance</h2>
<p>This section details processing and conformance requirements on XLink applications
and markup.</p>
<p>[<a name="dt-must" title="Must, May, etc.">Definition</a>: The key words <b>must</b>, <b>must
not</b>, <b>required</b>, <b>shall</b>, <b>shall not</b>, <b>should</b>, <b>should
not</b>, <b>recommended</b>, <b>may</b>, and <b>optional</b>
in this specification are to be interpreted as described in <a href="#rfc2119">[IETF RFC 2119]</a>.]</p>
<div class="div2">

<h3><a name="N1022"></a>3.1 Processing Dependencies</h3>
<p>XLink processing depends on <a href="#XML">[XML]</a>, <a href="#xname">[XML Names]</a>, <a href="#xbase">[XML Base]</a>, and <a href="#rfc2396">[IETF RFC 2396]</a> (as updated by <a href="#rfc2732">[IETF RFC 2732]</a>).</p>
</div>
<div class="div2">

<h3><a name="markup-reqs"></a>3.2 Markup Conformance</h3>
<p>An XML element conforms to XLink if:</p>
<ol>
<li><p>it has a <code>type</code> attribute from the XLink namespace whose
value is one of "simple", "extended", "locator", "arc", "resource", "title", or "none", and</p></li>
<li><p>it adheres to the conformance constraints imposed by the chosen XLink
element type, as prescribed in this specification.</p></li>
</ol>
<p>This specification imposes no particular constraints on DTDs; conformance
applies only to elements and attributes.</p>
</div>
<div class="div2">

<h3><a name="app-reqs"></a>3.3 Application Conformance</h3>
<p>An XLink application is any software module that interprets well-formed
XML documents containing XLink elements and attributes, or XML information
sets <a href="#infoset">[XIS]</a> containing information items and properties corresponding
to XLink elements and attributes. (This document refers to elements and attributes,
but all specifications herein apply to their information set equivalents as
well.) Such an application is conforming if:</p>
<ol>
<li><p>it observes the mandatory conditions for applications ("must")
set forth in this specification, and</p></li>
<li><p>for any optional conditions ("should" and "may")
it chooses to observe, it observes them in the way prescribed, and</p></li>
<li><p>it performs markup conformance testing according to all the conformance
constraints appearing in this specification.</p></li>
</ol>
</div>
</div>
<div class="div1">

<h2><a name="att-method"></a>4 XLink Markup Design</h2>
<p>This section describes the design of XLink's markup vocabulary.</p>
<p>Link markup needs to be recognized reliably by XLink applications in order
to be traversed and handled properly. XLink uses the mechanism described in
the Namespaces in XML Recommendation <a href="#xname">[XML Names]</a> to accomplish recognition
of the constructs in the XLink vocabulary.</p>
<p>The XLink namespace defined by this specification has the following URI:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>http://www.w3.org/1999/xlink</pre></td></tr></table>
<p>As dictated by <a href="#xname">[XML Names]</a>, the use of XLink elements and attributes
requires declaration of the XLink namespace. For example, the following declaration
would make the prefix <code>xlink</code> available within the <code>myElement</code>
element to represent the XLink namespace:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;myElement
  xmlns:xlink="http://www.w3.org/1999/xlink"&gt;
  ...
&lt;/myElement&gt;</pre></td></tr></table>
<div class="note"><p class="prefix"><b>Note:</b></p>
<p>Most code examples in this specification do not show an XLink namespace
declaration. The <code>xlink</code> prefix is used throughout to stand for the
declaration of the XLink namespace on elements in whose scope the so-marked
attribute appears (on the same element that bears the attribute or on some
ancestor element), whether or not an XLink namespace declaration is present
in the example.</p>
</div>
<p>XLink's namespace provides <b>global attributes</b> for use on elements
that are in any arbitrary namespace. The global attributes are <code>type</code>, <code>href</code>, <code>role</code>, <code>arcrole</code>, <code>title</code>, <code>show</code>, <code>actuate</code>, <code>label</code>, <code>from</code>,
and <code>to</code>. Document creators use the XLink global attributes to make
the elements in their own namespace, or even in a namespace they do not control,
recognizable as XLink elements. The <code>type</code> attribute indicates the
XLink element type (simple, extended, locator, arc, resource, or title); the
element type dictates the XLink-imposed constraints that such an element <a title="Must, May, etc." href="#dt-must">must</a> follow and the behavior of XLink applications
on encountering the element.</p>
<p>Following is an example of a <code>crossReference</code> element from a non-XLink
namespace that has XLink global attributes:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;my:crossReference
  xmlns:my="http://example.com/"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  xlink:type="simple"
  xlink:href="students.xml"
  xlink:role="http://www.example.com/linkprops/studentlist"
  xlink:title="Student List"
  xlink:show="new"
  xlink:actuate="onRequest"&gt;
Current List of Students
&lt;/my:crossReference&gt;</pre></td></tr></table>
<p>Using global attributes always requires the use of namespace prefixes on
the individual attributes and the use of the <code>type</code> attribute on
the element.</p>
<div class="div2">

<h3><a name="N1238"></a>4.1 XLink Attribute Usage Patterns</h3>
<p>While the XLink attributes are considered global by virtue of their use
of the namespace mechanism, their allowed combinations on any one XLink element
type depend greatly on the value of the special <code>type</code> attribute
(see <a href="#link-types"><b>5.3 XLink Element Type Attribute (type)</b></a> for more information) for the element on
which they appear. The conformance constraint notes in this specification
detail their allowed usage patterns. Following is a summary of the element
types (columns) on which the global attributes (rows) are allowed, with an
indication of whether a value is required (R) or optional (O):</p>
<table border="1" frame="border"><thead><tr><th rowspan="1" colspan="1"></th><th rowspan="1" colspan="1"><code>simple</code></th>
<th rowspan="1" colspan="1"><code>extended</code></th><th rowspan="1" colspan="1"><code>locator</code></th><th rowspan="1" colspan="1"><code>arc</code></th><th rowspan="1" colspan="1"><code>resource</code></th>
<th rowspan="1" colspan="1"><code>title</code></th></tr></thead><tbody><tr><td rowspan="1" colspan="1"><code>type</code></td><td rowspan="1" colspan="1">R</td>
<td rowspan="1" colspan="1">R</td><td rowspan="1" colspan="1">R</td><td rowspan="1" colspan="1">R</td><td rowspan="1" colspan="1">R</td><td rowspan="1" colspan="1">R</td></tr><tr><td rowspan="1" colspan="1"><code>href</code></td>
<td rowspan="1" colspan="1">O </td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">R</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>role</code></td>
<td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>arcrole</code></td>
<td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>title</code></td>
<td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>show</code></td>
<td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>actuate</code></td>
<td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>label</code></td>
<td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>from</code></td>
<td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>to</code></td>
<td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr></tbody></table>
<p>(See also <a href="#sample-dtd-appx"><b>B Sample DTD</b></a> for a non-normative DTD that
illustrates the allowed patterns of attributes.)</p>
<p>This specification uses the convention "<var>xxx</var>-type element"
to refer to elements that <a title="Must, May, etc." href="#dt-must">must</a> adhere to
a named set of constraints associated with an XLink element type, no matter
what name the element actually has. For example, "<code>locator</code>-type
element" would refer to all of the following elements:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;locator xlink:type="locator" ... /&gt;
&lt;loc xlink:type="locator" ... /&gt;
&lt;my:pointer xlink:type="locator" ... /&gt;</pre></td></tr></table>
</div>
<div class="div2">

<h3><a name="N1599"></a>4.2 XLink Element Type Relationships</h3>
<p>Various XLink element types have special meanings dictated by this specification
when they appear as direct children of other XLink element types. Following
is a summary of the child element types that play a significant role in particular
parent element types. (Other combinations have no XLink-dictated significance.)</p>
<table border="1"><thead><tr><th rowspan="1" colspan="1">Parent type</th><th rowspan="1" colspan="1">Significant child types</th>
</tr></thead><tbody><tr><td rowspan="1" colspan="1"><code>simple</code></td><td rowspan="1" colspan="1">none</td></tr><tr><td rowspan="1" colspan="1"><code>extended</code></td>
<td rowspan="1" colspan="1"><code>locator</code>, <code>arc</code>, <code>resource</code>, <code>title</code></td>
</tr><tr><td rowspan="1" colspan="1"><code>locator</code></td><td rowspan="1" colspan="1"><code>title</code></td></tr><tr><td rowspan="1" colspan="1"><code>arc</code></td>
<td rowspan="1" colspan="1"><code>title</code></td></tr><tr><td rowspan="1" colspan="1"><code>resource</code></td><td rowspan="1" colspan="1">none</td></tr>
<tr><td rowspan="1" colspan="1"><code>title</code></td><td rowspan="1" colspan="1">none</td></tr></tbody></table></div>
<div class="div2">

<h3><a name="defaulting"></a>4.3 Attribute Value Defaulting</h3>
<p>Using XLink potentially involves using a large number of attributes for
supplying important link information. In cases where the values of the desired
XLink attributes are unchanging across individual instances in all the documents
of a certain type, attribute value defaults (fixed or not) <a title="Must, May, etc." href="#dt-must">may</a>
be added to a DTD so that the attributes do not have to appear physically
on element start-tags. For example, if attribute defaults were provided for
the <code>xmlns:xlink</code>, <code>xmlns:my</code>, <code>type</code>, <code>show</code>,
and <code>actuate</code> attributes in the example in the introduction to <a href="#att-method"><b>4 XLink Markup Design</b></a>, the example would look as follows:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;my:crossReference
  xlink:href="students.xml"
  xlink:role="http://www.example.com/linkprops/studentlist"
  xlink:title="Student List"&gt;
Current List of Students
&lt;/my:crossReference&gt;</pre></td></tr></table>
<p>Information sets that have been created under the control of a DTD have
all attribute values filled in.</p>
</div>
<div class="div2">

<h3><a name="integrating"></a>4.4 Integrating XLink Usage with Other Markup</h3>
<p>This specification defines only attributes and attribute values in the
XLink namespace. There is no restriction on using non-XLink attributes alongside
XLink attributes. In addition, most XLink attributes are optional and the
choice of simple or extended link is up to the markup designer or document
creator, so a DTD that uses XLink features need not use or declare the entire
set of XLink's attributes. Finally, while this specification identifies the
minimum constraints on XLink markup, DTDs that use XLink are free to tighten
these constraints. The use of XLink does not absolve a valid document from
conforming to the constraints expressed in its governing DTD.</p>
<p>Following is an example of a <code>crossReference</code> element with both
XLink and non-XLink attributes:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;my:crossReference
  xmlns:my="http://example.com/"
  my:lastEdited="2000-06-10"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  xlink:type="simple"
  xlink:href="students.xml"&gt;
Current List of Students
&lt;/my:crossReference&gt;</pre></td></tr></table>
</div>
<div class="div2">

<h3><a name="legacy"></a>4.5 Using XLink with Legacy Markup</h3>
<p>Because XLink's global attributes require the use of namespace prefixes,
non-XLink-based links in legacy documents generally do not serve as conforming
XLink constructs as they stand, even if attribute value defaulting is used.
For example, XHTML 1.0 has an <code>a</code> element with an <code>href</code> attribute,
but because the attribute is a local one attached to the <code>a</code> element
in the XHTML namespace, it is not the same as an <code>xlink:href</code> global
attribute in the XLink namespace.</p>
</div>
</div>
<div class="div1">

<h2><a name="linking-elements"></a>5 XLink Elements and Attributes</h2>
<p>XLink offers two kinds of links:</p>
<dl>
<dt class="label">Extended links</dt>
<dd>
<p>Extended links offer full XLink functionality, such as inbound and third-party
arcs, as well as links that have arbitrary numbers of participating resources.
As a result, their structure can be fairly complex, including elements for
pointing to remote resources, elements for containing local resources, elements
for specifying arc traversal rules, and elements for specifying human-readable
resource and arc titles.</p>
<p>XLink defines a way to give an extended link special semantics for finding
linkbases; used in this fashion, an extended link helps an XLink application
process other links.</p>
</dd>
<dt class="label">Simple links</dt>
<dd>
<p>Simple links offer shorthand syntax for a common kind of link, an outbound
link with exactly two participating resources (into which category HTML-style <code>A</code>
and <code>IMG</code> links fall). Because simple links offer less functionality
than extended links, they have no special internal structure.</p>
<p>While simple links are conceptually a subset of extended links, they are
syntactically different. For example, to convert a simple link into an extended
link, several structural changes would be needed.</p>
</dd>
</dl>
<p>The following sections define the XLink elements and attributes.</p>
<div class="div2">

<h3><a name="extended-link"></a>5.1 Extended Links (<code>extended</code>-Type Element)</h3>
<p>[<a name="dt-extendedlink" title="Extended link">Definition</a>: An <b>extended link</b>
is a link that associates an arbitrary number of resources. The participating
resources <a title="Must, May, etc." href="#dt-must">may</a> be any combination of remote
and local.]</p>
<p>The only kind of link that is able to have inbound and third-party arcs
is an extended link. Typically, extended linking elements are stored separately
from the resources they associate (for example, in entirely different documents).
Thus, extended links are important for situations where the participating
resources are read-only, or where it is expensive to modify and update them
but inexpensive to modify and update a separate linking element, or where
the resources are in formats with no native support for embedded links (such
as many multimedia formats).</p>
<p>The following diagram shows an extended link that associates five remote
resources. This could represent, for example, information about a student's
course load: one resource being a description of the student, another being
a description of the student's academic advisor, two resources representing
courses that the student is attending, and the last resource representing
a course that the student is auditing.</p>
<img src="extended-ool.gif" alt="Stylized Diagram of Out-of-Line Extended Link">
<p>Without the extended link, the resources might be entirely unrelated; for
example, they might be in five separate documents. The lines emanating from
the extended link represent the association it creates among the resources.
However, notice that the lines do not have directionality. Directionality
is expressed with traversal rules; without such rules being provided, the
resources are associated in no particular order, with no implication as to
whether and how individual resources are accessed.</p>
<p>The following diagram shows an extended link that associates five remote
resources and one local resource (a special element inside the extended link
element). This could represent the same sort of course-load example as described
above, with the addition of the student's grade point average stored locally.
Again, the lines represent mere association of the six resources, without
traversal directions or behaviors implied.</p>
<img src="extended-inl.gif" alt="Stylized Diagram of Inline Extended Link">
<p>The XLink element type for extended links is any element with an attribute
in the XLink namespace called <code>type</code> with a value of "extended".</p>
<p>The <code>extended</code>-type element <a title="Must, May, etc." href="#dt-must">may</a>
contain a mixture of the following elements in any order, possibly along with
other content and markup:</p>
<ul>
<li><p><code>locator</code>-type elements that address the remote resources
participating in the link</p></li>
<li><p><code>arc</code>-type elements that provide traversal rules among the
link's participating resources</p></li>
<li><p><code>title</code>-type elements that provide human-readable labels for
the link</p></li>
<li><p><code>resource</code>-type elements that supply local resources that
participate in the link</p></li>
</ul>
<p>It is not an error for an <code>extended</code>-type element to associate fewer
than two resources. If the link has only one participating resource, or none
at all, it is simply untraversable. Such a link may still be useful, for example,
to associate properties with a single resource by means of XLink attributes,
or to provide a placeholder for link information that will be populated eventually.</p>
<p>Subelements of the <code>simple</code> or <code>extended</code> type anywhere inside
a parent <code>extended</code>-type element have no XLink-specified meaning. Subelements of the <code>locator</code>, <code>arc</code>, or <code>resource</code>
type that are not direct children of an <code>extended</code>-type element have
no XLink-specified meaning.</p>
<p>The <code>extended</code>-type element <a title="Must, May, etc." href="#dt-must">may</a>
have the semantic attributes <code>role</code> and <code>title</code> (see <a href="#link-semantics"><b>5.5 Semantic Attributes (role, arcrole, and title)</b></a>). They supply semantic information about the link as
a whole; the <code>role</code> attribute indicates a property that the entire
link has, and the <code>title</code> attribute indicates a human-readable description
of the entire link. If other XLink attributes are present on the element,
they have no XLink-specified relationship to the link. If both a <code>title</code>
attribute and one or more <code>title</code>-type elements are present, they have
no XLink-specified relationship; a higher-level application built on XLink
will likely want to specify appropriate treatment (for example, precedence)
in this case.</p>
<div class="example">

<h5>Example: Sample <code>extended</code>-Type Element Declarations and Instance</h5>
<p>Following is a non-normative set of declarations for an <code>extended</code>-type
element and its subelements. Parts of this example are reused throughout this
specification. Note that the <code>type</code> attribute and some other attributes
are defaulted in the DTD in order to highlight the attributes that are changing
on a per-instance basis.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ELEMENT courseload ((tooltip|person|course|gpa|go)*)&gt;
&lt;!ATTLIST courseload
  xmlns:xlink     CDATA           #FIXED "http://www.w3.org/1999/xlink"
  xlink:type      (extended)      #FIXED "extended"
  xlink:role      CDATA           #IMPLIED
  xlink:title     CDATA           #IMPLIED&gt;

&lt;!ELEMENT tooltip ANY&gt;
&lt;!ATTLIST tooltip
  xlink:type      (title)         #FIXED "title"
  xml:lang        CDATA           #IMPLIED&gt;

&lt;!ELEMENT person EMPTY&gt;
&lt;!ATTLIST person
  xlink:type      (locator)       #FIXED "locator"
  xlink:href      CDATA           #REQUIRED
  xlink:role      CDATA           #IMPLIED
  xlink:title     CDATA           #IMPLIED
  xlink:label     NMTOKEN         #IMPLIED&gt;

&lt;!ELEMENT course EMPTY&gt;
&lt;!ATTLIST course
  xlink:type      (locator)       #FIXED "locator"
  xlink:href      CDATA           #REQUIRED
  xlink:role      CDATA           #FIXED "http://www.example.com/linkprops/course"
  xlink:title     CDATA           #IMPLIED
  xlink:label     NMTOKEN         #IMPLIED&gt;

&lt;!-- GPA = "grade point average" --&gt;
&lt;!ELEMENT gpa ANY&gt;
&lt;!ATTLIST gpa
  xlink:type      (resource)      #FIXED "resource"
  xlink:role      CDATA           #FIXED "http://www.example.com/linkprops/gpa"
  xlink:title     CDATA           #IMPLIED
  xlink:label     NMTOKEN         #IMPLIED&gt;

&lt;!ELEMENT go EMPTY&gt;
&lt;!ATTLIST go
  xlink:type      (arc)           #FIXED "arc"
  xlink:arcrole   CDATA           #IMPLIED
  xlink:title     CDATA           #IMPLIED
  xlink:show      (new
                  |replace
                  |embed
                  |other
                  |none)          #IMPLIED
  xlink:actuate   (onLoad
                  |onRequest
                  |other
                  |none)          #IMPLIED
  xlink:from      NMTOKEN         #IMPLIED
  xlink:to        NMTOKEN         #IMPLIED&gt;</pre></td></tr></table>
<p>Following is how XML elements using these declarations might look.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;courseload&gt;

  &lt;tooltip&gt;Course Load for Pat Jones&lt;/tooltip&gt;

  &lt;person
    xlink:href="students/patjones62.xml"
    xlink:label="student62"
    xlink:role="http://www.example.com/linkprops/student"
    xlink:title="Pat Jones" /&gt;

  &lt;person
    xlink:href="profs/jaysmith7.xml"
    xlink:label="prof7"
    xlink:role="http://www.example.com/linkprops/professor"
    xlink:title="Dr. Jay Smith" /&gt;
  &lt;!-- more remote resources for professors, teaching assistants, etc. --&gt;

  &lt;course
    xlink:href="courses/cs101.xml"
    xlink:label="CS-101"
    xlink:title="Computer Science 101" /&gt;
  &lt;!-- more remote resources for courses, seminars, etc. --&gt;

  &lt;gpa xlink:label="PatJonesGPA"&gt;3.5&lt;/gpa&gt;

  &lt;go
    xlink:from="student62"
    xlink:to="PatJonesGPA"
    xlink:show="new"
    xlink:actuate="onRequest"
    xlink:title="Pat Jones's GPA" /&gt;
  &lt;go
    xlink:from="CS-101"
    xlink:arcrole="http://www.example.com/linkprops/auditor"
    xlink:to="student62"
    xlink:show="replace"
    xlink:actuate="onRequest"
    xlink:title="Pat Jones, auditing the course" /&gt;
  &lt;go
    xlink:from="student62"
    xlink:arcrole="http://www.example.com/linkprops/advisor"
    xlink:to="prof7"
    xlink:show="replace"
    xlink:actuate="onRequest"
    xlink:title="Dr. Jay Smith, advisor" /&gt;

&lt;/courseload&gt;</pre></td></tr></table>
</div>
<div class="div3">

<h4><a name="local-resource"></a>5.1.1 Local Resources for an Extended Link  (<code>resource</code>-Type Element)</h4>
<p>An extended link indicates its participating local resources by means of
special subelements that appear inside the extended link. An entire subelement,
together with all of its contents, makes up a local resource.</p>
<p>The XLink element for local resources is any element with an attribute
in the XLink namespace called <code>type</code> with a value of "resource".</p>
<p>The <code>resource</code>-type element <a title="Must, May, etc." href="#dt-must">may</a>
have any content; whatever content is present has no XLink-specified relationship
to the link. It is possible for a <code>resource</code>-type element to have no
content; in cases where it serves as a starting resource expected to be traversed
on request, interactive XLink applications will typically generate some content
in order to give the user a way to initiate the traversal.  If a <code>resource</code>-type element has anything
other than an <code>extended</code>-type element for a parent, the <code>resource</code>-type
element has no XLink-specified meaning.</p>
<p>The <code>resource</code>-type element <a title="Must, May, etc." href="#dt-must">may</a>
have the semantic attributes <code>role</code> and <code>title</code> (see <a href="#link-semantics"><b>5.5 Semantic Attributes (role, arcrole, and title)</b></a>) and the traversal attribute <code>label</code> (see <a href="#traversal-atts"><b>5.7 Traversal Attributes (label, from, and to)</b></a>). The semantic attributes supply information about
the resource in generic terms, outside of the context of a particular arc
that leads to it; the <code>role</code> attribute indicates a property of the
resource, and the <code>title</code> attribute indicates a human-readable description
of the resource. The <code>label</code> attribute provides a way for an <code>arc</code>-type
element to refer to it in creating a traversal arc.</p>
<div class="example">

<h5>Example: Sample <code>resource</code>-Type Element Declarations and Instance</h5>
<p>Following is a non-normative set of declarations for a <code>resource</code>-type
element.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ELEMENT gpa ANY&gt;
&lt;!ATTLIST gpa
  xlink:type      (resource)      #FIXED "resource"
  xlink:role      CDATA           #FIXED "http://www.example.com/linkprops/gpa"
  xlink:title     CDATA           #IMPLIED
  xlink:label     NMTOKEN         #IMPLIED&gt;</pre></td></tr></table>
<p>Following is how an XML element using these declarations might look.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>  &lt;gpa xlink:label="PatJonesGPA"&gt;3.5&lt;/gpa&gt;
</pre></td></tr></table>
</div></div>
<div class="div3">

<h4><a name="remote-resource"></a>5.1.2 Remote Resources for an Extended Link (<code>locator</code>-Type Element)</h4>
<p>An extended link indicates remote resources that participate in it by means
of locator elements.</p>
<p>The XLink element for locators is any element with an attribute in the
XLink namespace called <code>type</code> with a value of "locator".</p>
<p>The <code>locator</code>-type element <a title="Must, May, etc." href="#dt-must">may</a>
have any content. Other than <code>title</code>-type elements that are direct
children (see <a href="#title-element"><b>5.1.4 Titles for Extended Links, Locators, and Arcs (title-Type Element)</b></a>), whatever content is present
has no XLink-specified relationship to the link. If a <code>locator</code>-type
element contains nested XLink elements, such contained elements have no XLink-specified
relationship to the parent link. If a <code>locator</code>-type element has anything
other than an <code>extended</code>-type element for a parent, the <code>locator</code>-type
element has no XLink-specified meaning.</p>
<div class="constraint">
<p class="prefix"><a name="cn-locator-atts"></a><b>Constraint: Attributes on Locator Element</b></p>
<p>The <code>locator</code>-type element <a title="Must, May, etc." href="#dt-must">must</a>
have the locator attribute (see <a href="#link-locators"><b>5.4 Locator Attribute (href)</b></a>). The locator
attribute (<code>href</code>) <a title="Must, May, etc." href="#dt-must">must</a> have a value
supplied.</p>
</div>
<p>The <code>locator</code>-type element <a title="Must, May, etc." href="#dt-must">may</a>
have the semantic attributes <code>role</code> and <code>title</code> (see <a href="#link-semantics"><b>5.5 Semantic Attributes (role, arcrole, and title)</b></a>) and the traversal attribute <code>label</code> (see <a href="#traversal-atts"><b>5.7 Traversal Attributes (label, from, and to)</b></a>). The locator attribute provides a URI reference that
identifies a remote resource. The semantic attributes supply information about
the resource in generic terms, outside of the context of a particular arc
that leads to it; the <code>role</code> attribute indicates a property that
the resource has, and the <code>title</code> attribute indicates a human-readable
description of the resource. The <code>label</code> attribute provides a way
for an <code>arc</code>-type element to refer to it in creating a traversal arc.</p>
<div class="note"><p class="prefix"><b>Note:</b></p>
<p>A <code>locator</code>-type element, by itself, does not constitute a link
just because it has a locator (<code>href</code>) attribute; unlike a <code>simple</code>-type
element, it does not create an XLink-governed association between itself and
the referenced resource.</p>
</div>
<div class="example">

<h5>Example: Sample <code>locator</code>-Type Element Declarations and Instance</h5>
<p>Following is a non-normative set of declarations for a <code>locator</code>-type
element.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ELEMENT person EMPTY&gt;
&lt;!ATTLIST person
  xlink:type      (locator)       #FIXED "locator"
  xlink:href      CDATA           #REQUIRED
  xlink:role      CDATA           #IMPLIED
  xlink:title     CDATA           #IMPLIED
  xlink:label     NMTOKEN         #IMPLIED&gt;

&lt;!ELEMENT course EMPTY&gt;
&lt;!ATTLIST course
  xlink:type      (locator)       #FIXED "locator"
  xlink:href      CDATA           #REQUIRED
  xlink:role      CDATA           #FIXED "http://www.example.com/linkprops/course"
  xlink:title     CDATA           #IMPLIED
  xlink:label     NMTOKEN         #IMPLIED&gt;</pre></td></tr></table>
<p>Following is how XML elements using these declarations might look.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;person
  xlink:href="students/patjones62.xml"
  xlink:label="student62"
  xlink:role="http://www.example.com/linkprops/student"
xlink:title="Pat Jones" /&gt;

&lt;person
  xlink:href="profs/jaysmith7.xml"
  xlink:label="prof7"
  xlink:role="http://www.example.com/linkprops/professor"
  xlink:title="Dr. Jay Smith" /&gt;

&lt;course
  xlink:href="courses/cs101.xml"
  xlink:label="CS-101"
  xlink:title="Computer Science 101" /&gt;</pre></td></tr></table>
</div></div>
<div class="div3">

<h4><a name="xlink-arcs"></a>5.1.3 Traversal Rules for an Extended Link  (<code>arc</code>-Type Element)</h4>
<p>An extended link <a title="Must, May, etc." href="#dt-must">may</a> indicate rules for
traversing among its participating resources by means of a series of optional
arc elements.</p>
<p>The XLink element for arcs is any element with an attribute in the XLink
namespace called <code>type</code> with a value of "arc".</p>
<p>The <code>arc</code>-type element <a title="Must, May, etc." href="#dt-must">may</a> have
any content. Other than <code>title</code>-type elements that are direct children
(see <a href="#title-element"><b>5.1.4 Titles for Extended Links, Locators, and Arcs (title-Type Element)</b></a>), whatever content is present has no XLink-specified
relationship to the link. If an <code>arc</code>-type element has anything other
than an <code>extended</code>-type element for its parent, the <code>arc</code>-type
element has no XLink-specified meaning.</p>
<p>The <code>arc</code>-type element <a title="Must, May, etc." href="#dt-must">may</a> have
the traversal attributes <code>from</code> and <code>to</code> (see <a href="#traversal-atts"><b>5.7 Traversal Attributes (label, from, and to)</b></a>),
the behavior attributes <code>show</code> and <code>actuate</code> (see <a href="#link-behaviors"><b>5.6 Behavior Attributes (show and actuate) </b></a>) and the semantic attributes <code>arcrole</code> and <code>title</code>
(see <a href="#link-semantics"><b>5.5 Semantic Attributes (role, arcrole, and title)</b></a>).</p>
<p>The traversal attributes define the desired traversal between pairs of
resources that participate in the same link, where the resources are identified
by their <code>label</code> attribute values. The <code>from</code> attribute
defines resources from which traversal <a title="Must, May, etc." href="#dt-must">may</a>
be initiated, that is, <a title="Starting resource" href="#dt-starting-resource">starting resources</a>,
while the <code>to</code> attribute defines resources that <a title="Must, May, etc." href="#dt-must">may</a>
be traversed to, that is, <a title="Ending resource" href="#dt-ending-resource">ending resources</a>.
The behavior attributes specify the desired behavior for XLink applications
to use when traversing to the ending resource.</p>
<p>The semantic attributes describe the meaning of the arc's ending resource
relative to its starting resource. The <code>arcrole</code> attribute corresponds
to the <a href="#rdf">[RDF]</a> notion of a property, where the role can be interpreted
as stating that "<var>starting-resource</var> HAS <var>arc-role</var> <var>ending-resource</var>."
This contextual role can differ from the meaning of an ending resource when
taken outside the context of this particular arc. For example, a resource
might generically represent a "person," but in the context of
a particular arc it might have the role of "mother" and in the
context of a different arc it might have the role of "daughter."</p>
<p>When the same resource serves as a starting resource in several arcs (whether
in a single link or across many links), traversal-request behavior is unconstrained
by this specification, but one possibility for interactive applications is
a pop-up menu that lists the relevant arc or link titles.</p>
<p>The following diagram shows an extended link that associates five remote
resources and provides rules for traversal among them. All of the arcs specified
are third-party arcs; that is, the arcs go exclusively between remote resources.
The nondirectional solid lines indicate, as before, that the link is associating
the five resources; the new dotted arrows indicate the traversal rules that
the link provides. Notice that some resources share the same <code>label</code>
value.</p>
<img src="extended-arcs.gif" alt="Stylized Diagram of Out-of-Line Extended Link with Arcs">
<p>This diagram reflects directional traversal arcs created by the following
settings, where both As and Cs are allowed to initiate traversal to all Bs.
Because some labels appear on several resources, each arc specification potentially
creates several traversal arcs at once:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;go xlink:type="arc" xlink:from="A" xlink:to="B" /&gt;
&lt;go xlink:type="arc" xlink:from="C" xlink:to="B" /&gt;</pre></td></tr></table>
<p>As another example, assume an extended link that contains five locators,
two with <code>label</code> values of "parent" and three with <code>label</code>
values of "child":</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;extendedlink xlink:type="extended"&gt;
  &lt;loc xlink:type="locator" xlink:href="..." xlink:label="parent" xlink:title="p1" /&gt;
  &lt;loc xlink:type="locator" xlink:href="..." xlink:label="parent" xlink:title="p2" /&gt;
  &lt;loc xlink:type="locator" xlink:href="..." xlink:label="child"  xlink:title="c1" /&gt;
  &lt;loc xlink:type="locator" xlink:href="..." xlink:label="child"  xlink:title="c2" /&gt;
  &lt;loc xlink:type="locator" xlink:href="..." xlink:label="child"  xlink:title="c3" /&gt;
... &lt;!-- arc-type elements would go here --&gt;
&lt;/extendedlink&gt;</pre></td></tr></table>
<p>The following specifies traversal from parent resources to child resources,
which includes all of p1-c1, p1-c2, p1-c3, p2-c1, p2-c2, and p2-c3:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;go xlink:type="arc" xlink:from="parent" xlink:to="child" /&gt;</pre></td></tr></table>
<p>If no value is supplied for a <code>from</code> or <code>to</code> attribute,
the missing value is interpreted as standing for <em>all</em> the labels
supplied on <code>locator</code>-type elements in that <code>extended</code>-type
element. For example, the following specifies traversal from parents to children
and also from children to children, which includes all of p1-c1, p1-c2, p1-c3,
p2-c1, p2-c2, p2-c3, c1-c1, c1-c2, c1-c3, c2-c1, c2-c2, c2-c3, c3-c1, c3-c2,
and c3-c3:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;go xlink:type="arc" xlink:to="child" /&gt;</pre></td></tr></table>
<p>In this case, note that the traversal rules include arcs from some resources
to other resources with the same label (from children to other children),
as well as from some resources to themselves (from a child to itself); this
is not an error.</p>
<p>If no <code>arc</code>-type elements are provided in an extended link, then
by extension the missing <code>from</code> and <code>to</code> values are interpreted
as standing for all the labels in that link. This would be equivalent to the
following traversal specification:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;go xlink:type="arc" /&gt;</pre></td></tr></table>
<p>When more than one locator has the same label, the set of locators with
the same label are to be understood as individual locators, rather than as
referring to an aggregate resource; the traversal behavior of such a link
might be the same as for a link where all the locators have different roles
and the appropriate arcs are specified to produce the identical traversal
pairs.</p>
<p>If the arc traversal rules for an extended link leave out any possible
traversal pairs, XLink defines no traversal for these pairs. A higher-level
application <a title="Must, May, etc." href="#dt-must">may </a>perform non-XLink-directed
traversals; for example, a link-checking process might traverse all available
pairs of resources.</p>
<div class="constraint">
<p class="prefix"><a name="cn-arc-duplicates"></a><b>Constraint: No Arc Duplication</b></p>
<p>Each <code>arc</code>-type element <a title="Must, May, etc." href="#dt-must">must</a> have
a pair of <code>from</code> and <code>to</code> values that does not repeat the <code>from</code>
and <code>to</code> values (respectively) for any other <code>arc</code>-type element
in the same extended link; that is, each pair in a link <a title="Must, May, etc." href="#dt-must">must</a>
be unique.</p>
</div>
<div class="example">

<h5>Example: Sample <code>arc</code>-Type Element Declarations and Instance</h5>
<p>Following is a non-normative set of declarations for an <code>arc</code>-type
element.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ELEMENT go EMPTY&gt;
&lt;!ATTLIST go
  xlink:type      (arc)           #FIXED "arc"
  xlink:arcrole   CDATA           #IMPLIED
  xlink:title     CDATA           #IMPLIED
  xlink:show      (new
                  |replace
                  |embed
                  |other
                  |none)          #IMPLIED
  xlink:actuate   (onLoad
                  |onRequest
                  |other
                  |none)          #IMPLIED
  xlink:from      NMTOKEN         #IMPLIED
  xlink:to        NMTOKEN         #IMPLIED&gt;</pre></td></tr></table>
<p>Following is how XML elements using these declarations might look.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;go
  xlink:from="student62"
  xlink:to="PatJonesGPA"
  xlink:show="new"
  xlink:actuate="onRequest"
  xlink:title="Pat Jones's GPA" /&gt;
&lt;go
  xlink:from="CS-101"
  xlink:arcrole="http://www.example.com/linkprops/auditor"
  xlink:to="student62"
  xlink:show="replace"
  xlink:actuate="onRequest"
  xlink:title="Pat Jones, auditing the course" /&gt;
&lt;go
  xlink:from="student62"
  xlink:arcrole="http://www.example.com/linkprops/advisor"
  xlink:to="prof7"
  xlink:show="replace"
  xlink:actuate="onRequest"
  xlink:title="Dr. Jay Smith, advisor" /&gt;</pre></td></tr></table>
</div></div>
<div class="div3">

<h4><a name="title-element"></a>5.1.4 Titles for Extended Links, Locators, and Arcs (<code>title</code>-Type Element)</h4>
<p>The <code>extended</code>-, <code>locator</code>-, and <code>arc</code>-type elements <a title="Must, May, etc." href="#dt-must">may</a> have the <code>title</code> attribute (more about
which see <a href="#link-semantics"><b>5.5 Semantic Attributes (role, arcrole, and title)</b></a>). However, they <a title="Must, May, etc." href="#dt-must">may</a>
also have a series of one or more <code>title</code>-type elements. Such elements
are useful, for example, for cases where human-readable label information
needs further element markup, or where multiple titles are necessary. One
common motivation for using the <code>title</code>-type element is to account
for internationalization and localization. For example, title markup might
be necessary for bidirectional contexts or in East Asian languages, and multiple
titles might be necessary for different natural-language versions of a title.</p>
<p>The XLink element for titles is any element with an attribute in the XLink
namespace called <code>type</code> with a value of "title".</p>
<p>The <code>title</code>-type element <a title="Must, May, etc." href="#dt-must">may</a> have
any content. If a <code>title</code>-type element contains nested XLink elements,
such contained elements have no XLink-specified relationship to the parent
link containing the title. If a <code>title</code>-type element has anything other
than an <code>extended</code>-, <code>locator</code>-, or <code>arc</code>-type element
for a parent, the <code>title</code>-type element has no XLink-specified meaning.</p>
<div class="example">

<h5>Example: Sample <code>title</code>-Type Element Declarations and Instance</h5>
<p>Following is a non-normative set of declarations for a <code>title</code>-type
element. The element has been given the <code>xml:lang</code> attribute, which <a title="Must, May, etc." href="#dt-must">may</a> be used in conjunction with server settings or
other contextual information in determining which title to present.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ELEMENT advisorname (name)&gt;
&lt;!ATTLIST advisorname
  xlink:type      (title)         #FIXED "title"
  xml:lang        CDATA           #IMPLIED&gt;

&lt;!ELEMENT name (honorific?, given, family)&gt;
&lt;!-- Further subelement declarations for names --&gt;</pre></td></tr></table>
<p>Following is how XML elements using these declarations might look.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;advisor xlink:href="profs/jaysmith7.xml" ...&gt;
  &lt;advisorname xml:lang="en"&gt;
    &lt;name&gt;
      &lt;honorific&gt;Dr.&lt;/honorific&gt;
      &lt;given&gt;Jay&lt;/given&gt;
      &lt;family&gt;Smith&lt;/family&gt;
    &lt;/name&gt;
  &lt;/advisorname&gt;
&lt;/advisor&gt;</pre></td></tr></table>
</div></div>
<div class="div3">

<h4><a name="xlg"></a>5.1.5 Locating Linkbases (Special Arc Role)</h4>
<p>For an XLink application to traverse from a starting resource to an ending
resource, it needs to locate both the starting resource and the link. Locating
the two pieces is not a problem in the case of outbound arcs because the starting
resource is either the linking element itself or a child of the linking element.
However, in the case of inbound and third-party arcs, the XLink application
needs to be able to find both pieces somehow.</p>
<p>In the course load example, extended links can associate pairs of remote
resources representing students and courses. In order for the system to load
and present a "student resource" (such as a description and picture
of the person) in a way that offers traversal to related information (for
example, by allowing users to click on the student's name to traverse to information
about the courses in which she is enrolled), it needs to locate and use the
extended links that contain the association.</p>
<p><a title="Linkbase" href="#dt-linkbase">Linkbases</a> are often used to make link
management easier by gathering together a number of related linking elements.
XLink provides a way to instruct XLink applications to access potentially
relevant linkbases. The instruction takes the form of an arc specification
(whether an explicit one in an extended link, or an implicit one in a simple
link) that has the following value for its arcrole attribute:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>http://www.w3.org/1999/xlink/properties/linkbase</pre></td></tr></table>
<div class="constraint">
<p class="prefix"><a name="require-xls-xml"></a><b>Constraint: Linkbases Must Be XML</b></p>
<p>Any linkbase specified as the ending resource of an arc with this special
value <a title="Must, May, etc." href="#dt-must">must</a> be an XML document.</p>
</div>
<p>(XLink applications <a title="Must, May, etc." href="#dt-must">may</a> also use any other
means to locate and process additional linkbases.)</p>
<p>The handling of a linkbase arc is much like the handling of a normal arc,
except that traversal entails loading the ending resource (the linkbase) to
extract its links for later use, rather than to present it to a user or to
perform some other processing. Its handling is also special in that XLink
applications <a title="Must, May, etc." href="#dt-must">must</a> suspend traversal of linkbase
arcs at user option.</p>
<p>Specifically, on loading a linkbase arc, an XLink application <a title="Must, May, etc." href="#dt-must">should</a> keep track of what the starting resource is.
Whenever a document containing that starting resource is loaded and traversal
of the linkbase arc is actuated, the application <a title="Must, May, etc." href="#dt-must">should</a>
access the linkbase and extract any extended links found inside it. In the
case that the extracted resource is a portion of a complete XML document,
such as a range or a string range, only those extended links completely contained
in the extracted portion <a title="Must, May, etc." href="#dt-must">should</a> be made available.</p>
<p>The timing of linkbase arc traversal depends on the value of the <code>actuate</code>
attribute on the arc. For example, if the value is "onLoad",
the linkbase is loaded and its links extracted as soon as the starting resource
is loaded. Any <code>show</code> attribute value on a linkbase arc <a title="Must, May, etc." href="#dt-must">must</a> be ignored, because traversal does not entail
presentation in this case.</p>
<p>Linkbases <a title="Must, May, etc." href="#dt-must">may</a> be chained by virtue of
serving as the starting resource of yet another linkbase arc. The application
interpreting an initial linkbase arc <a title="Must, May, etc." href="#dt-must">may</a>
choose to limit the number of steps processed in the chain.</p>
<p>An application <a title="Must, May, etc." href="#dt-must">should</a> maintain a list
of extended links retrieved as a result of processing a linkbase, and <a title="Must, May, etc." href="#dt-must">should not</a> retrieve duplicate resources or links in
the case where a cyclic dependency exists. To ease XLink processing, document
creators <a title="Must, May, etc." href="#dt-must">may</a> wish to define linkbase arcs
near the beginning of a document.</p>
<div class="example">

<h5>Example: Annotating a Specification</h5>
<p>Following is a non-normative set of declarations for an extended link that
specializes in providing linkbase arcs:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ELEMENT basesloaded ((startrsrc|linkbase|load)*)&gt;
&lt;!ATTLIST basesloaded
  xlink:type      (extended)      #FIXED "extended"&gt;

&lt;!ELEMENT startrsrc EMPTY&gt;
&lt;!ATTLIST startrsrc
  xlink:type      (locator)       #FIXED "locator"
  xlink:href      CDATA           #REQUIRED
  xlink:label     NMTOKEN         #IMPLIED&gt;

&lt;!ELEMENT linkbase EMPTY&gt;
&lt;!ATTLIST linkbase
  xlink:type      (locator)       #FIXED "locator"
  xlink:href      CDATA           #REQUIRED
  xlink:label     NMTOKEN         #IMPLIED&gt;

&lt;!ELEMENT load EMPTY&gt;
&lt;!ATTLIST load
  xlink:type      (arc)           #FIXED "arc"
  xlink:arcrole   CDATA           #FIXED "http://www.w3.org/1999/xlink/properties/linkbase"
  xlink:actuate   (onLoad
                  |onRequest
                  |other
                  |none)          #IMPLIED
  xlink:from      NMTOKEN         #IMPLIED
  xlink:to        NMTOKEN         #IMPLIED&gt;</pre></td></tr></table>
<p>Following is how an XML element using these declarations might look. This
would indicate that when a specification document is loaded, a linkbase full
of annotations to it <a title="Must, May, etc." href="#dt-must">should</a> automatically
be loaded as well, possibly necessitating re-rendering of the entire specification
document to reveal any regions within it that serve as starting resources
in the links found in the linkbase.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;basesloaded&gt;
  &lt;startrsrc xlink:label="spec" xlink:href="spec.xml" /&gt;
  &lt;linkbase xlink:label="linkbase" xlink:href="linkbase.xml" /&gt;
  &lt;load xlink:from="spec" xlink:to="linkbase" actuate="onLoad" /&gt;
&lt;/basesloaded&gt;</pre></td></tr></table>
<p>Following is how an XML element using these declarations might look if
the linkbase loading were on request. This time, the starting resource consists
of the words "Click here to reveal annotations." If the starting
resource were the entire document as in the example above, a reasonable behavior
for allowing a user to actuate traversal would be a confirmation dialog box.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;basesloaded&gt;
  &lt;startrsrc
    xlink:label="spec"
    xlink:href="spec.xml#string-range(//*,'Click here to reveal annotations.')" /&gt;
  &lt;linkbase xlink:label="linkbase" xlink:href="linkbase.xml" /&gt;
  &lt;load xlink:from="spec" xlink:to="linkbase" actuate="onRequest" /&gt;
&lt;/basesloaded&gt;</pre></td></tr></table>
</div></div>
</div>
<div class="div2">

<h3><a name="simple-links"></a>5.2 Simple Links (<code>simple</code>-Type Element)</h3>
<p>[<a name="dt-simplelink" title="Simple link">Definition</a>: A <b>simple link</b>
is a link that associates exactly two <a title="Resource" href="#dt-resource">resources</a>,
one <a title="Local resource" href="#dt-local-resource">local</a> and one <a title="Remote resource" href="#dt-remote-resource">remote</a>,
with an arc going from the former to the latter. Thus, a simple link is always
an outbound link.]</p>
<p>The purpose of a simple link is to be a convenient shorthand for the equivalent
extended link. A single simple linking element combines the basic functions
of an <code>extended</code>-type element, a <code>locator</code>-type element, an <code>arc</code>-type
element, and a <code>resource</code>-type element.</p>
<p>The following diagram shows the characteristics of a simple link; it associates
one local and one remote resource, and implicitly provides a single traversal
arc from the local resource to the remote one. This could represent, for example,
the name of a student appearing in text which, when clicked, leads to information
about the student.</p>
<img src="simple.gif" alt="Stylized Diagram of Simple Link"><div class="example">

<h5>Example: Simple Link Functionality Done with an Extended Link</h5>
<p>A simple link could be represented by an extended link in approximately
the following way:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;studentlink xlink:type="extended"&gt;
  &lt;resource
    xlink:type="resource"
    xlink:label="local"&gt;Pat Jones&lt;/resource&gt;
  &lt;locator
    xlink:type="locator"
    xlink:href="..."
    xlink:label="remote"
    xlink:role="..."
    xlink:title="..." /&gt;

  &lt;go
    xlink:type="arc"
    xlink:from="local"
    xlink:to="remote"
    xlink:arcrole="..."
    xlink:show="..."
    xlink:actuate="..." /&gt;
&lt;/studentlink&gt;</pre></td></tr></table>
</div>
<p>A simple link combines all the features above (except for the types and
labels) into a single element. In cases where only this subset of features
is required, the XLink simple linking element is available as an alternative
to the extended linking element. The features missing from simple links are
as follows:</p>
<ul>
<li><p>Supplying arbitrary numbers of local and remote resources</p></li>
<li><p>Specifying an arc from its remote resource to its local resource</p>
</li>
<li><p>Associating a title with the single hardwired arc</p></li>
<li><p>Associating a role or title with the local resource</p></li>
<li><p>Associating a role or title with the link as a whole</p></li>
</ul>
<p>The XLink element for simple links is any element with an attribute in
the XLink namespace called <code>type</code> with a value of "simple".
The simple equivalent of the above extended link would be as follows:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;studentlink xlink:href="..."&gt;Pat Jones&lt;/studentlink&gt;</pre></td></tr></table>
<p>The <code>simple</code>-type element <a title="Must, May, etc." href="#dt-must">may</a> have
any content. The <code>simple</code>-type element itself, together with all of
its content, is the local resource of the link, as if the element were a <code>resource</code>-type
element. If a <code>simple</code>-type element contains nested XLink elements,
such contained elements have no XLink-specified relationship to the parent
link. It is possible for a <code>simple</code>-type element to have no content;
in cases where the link is expected to be traversed on request, interactive
XLink applications will typically generate some content in order to give the
user a way to initiate the traversal.</p>
<p>The <code>simple</code>-type element effectively takes the locator attribute <code>href</code>
and the semantic attributes <code>role</code> and <code>title</code> from the <code>locator</code>-type
element, and the behavior attributes <code>show</code> and <code>actuate</code>
and the single semantic attribute <code>arcrole</code> from the <code>arc</code>-type
element.</p>
<p>It is not an error for a <code>simple</code>-type element to have no locator
(<code>href</code>) attribute value. If a value is not provided, the link is
simply untraversable. Such a link may still be useful, for example, to associate
properties with the resource by means of XLink attributes.</p>
<div class="example">

<h5>Example: Sample <code>simple</code>-Type Element Declarations and Instance</h5>
<p>Following is a non-normative set of declarations for a <code>simple</code>-type
element.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ELEMENT studentlink ANY&gt;
&lt;!ATTLIST studentlink
  xlink:type      (simple)        #FIXED "simple"
  xlink:href      CDATA           #IMPLIED
  xlink:role      NMTOKEN         #FIXED "http://www.example.com/linkprops/student"
  xlink:arcrole   CDATA           #IMPLIED
  xlink:title     CDATA           #IMPLIED
  xlink:show      (new
                  |replace
                  |embed
                  |other
                  |none)          #IMPLIED
  xlink:actuate   (onLoad
                  |onRequest
                  |other
                  |none)          #IMPLIED&gt;</pre></td></tr></table>
<p>Following is how an XML document might use these declarations.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>..., and &lt;studentlink xlink:href="students/patjones62.xml"&gt;Pat
Jones&lt;/studentlink&gt; is popular around the student union.</pre></td></tr></table>
</div></div>
<div class="div2">

<h3><a name="link-types"></a>5.3 XLink Element Type Attribute (<code>type</code>)</h3>
<p>The attribute that identifies XLink element types is <code>type</code>.</p>
<div class="constraint">
<p class="prefix"><a name="cn-type-value"></a><b>Constraint: type Value</b></p>
<p>The value of the <code>type</code> attribute <a title="Must, May, etc." href="#dt-must">must</a>
be supplied. The value <a title="Must, May, etc." href="#dt-must">must</a> be one of "simple", "extended", "locator", "arc", "resource", "title",
or "none".</p>
</div>
<p>When the value of the <code>type</code> attribute is "none",
the element has no XLink-specified meaning, and any XLink-related content
or attributes have no XLink-specified relationship to the element.</p>
<div class="example">

<h5>Example: Sample <code>type</code> Attribute Declarations</h5>
<p>Following is a non-normative attribute-list declaration for <code>type</code>
on an element intended to be <code>simple</code>-type.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ATTLIST xlink:simple
  xlink:type      (simple)        #FIXED "simple"
  ...&gt;</pre></td></tr></table>
<p>For an element that serves as an XLink element only on some occasions,
one declaration might be as follows, where the document creator sets the value
to "simple" in some circumstances and "none"
in others. The use of "none" might be useful in helping XLink
applications to avoid checking for the presence of an <code>href</code> value.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ATTLIST commandname
  xlink:type      (simple|none)   #REQUIRED
  xlink:href      CDATA           #IMPLIED&gt;</pre></td></tr></table>
</div></div>
<div class="div2">

<h3><a name="link-locators"></a>5.4 Locator Attribute (<code>href</code>)</h3>
<p>The attribute that supplies the data that allows an XLink application to
find a remote resource (or resource fragment) is <code>href</code>. It <a title="Must, May, etc." href="#dt-must">may</a> be used on <code>simple</code>-type elements, and <a title="Must, May, etc." href="#dt-must">must</a> be used on <code>locator</code>-type elements.</p>
<p>The value of the <code>href</code> attribute <a title="Must, May, etc." href="#dt-must">must</a>
be a URI reference as defined in <a href="#rfc2396">[IETF RFC 2396]</a>, or must result in
a URI reference after the escaping procedure described below is applied. The
procedure is applied when passing the URI reference to a URI resolver.</p>
<p>Some characters are disallowed in URI references, even if they are allowed
in XML; the disallowed characters include all non-ASCII characters, plus the
excluded characters listed in Section 2.4 of <a href="#rfc2396">[IETF RFC 2396]</a>, except
for the number sign (#) and percent sign (%) and the square bracket characters
re-allowed in <a href="#rfc2732">[IETF RFC 2732]</a>. Disallowed characters <a title="Must, May, etc." href="#dt-must">must</a>
be escaped as follows:</p>
<ol>
<li><p>Each disallowed character is converted to UTF-8 <a href="#rfc2279">[IETF RFC 2279]</a>
as one or more bytes.</p></li>
<li><p>Any bytes corresponding to a disallowed character are escaped with
the URI escaping mechanism (that is, converted to <code>%</code><var>HH</var>,
where HH is the hexadecimal notation of the byte value).</p></li>
<li><p>The original character is replaced by the resulting character sequence.</p>
</li>
</ol>
<p>Because it is impractical for any application to check that a value is
a URI reference, this specification follows the lead of <a href="#rfc2396">[IETF RFC 2396]</a>
in this matter and imposes no such conformance testing requirement on XLink
applications.</p>
<p>If the URI reference is relative, its absolute version <a title="Must, May, etc." href="#dt-must">must</a>
be computed by the method of <a href="#xbase">[XML Base]</a> before use.</p>
<p>For locators into XML resources, the format of the fragment identifier
(if any) used within the URI reference is specified by the XPointer specification <a href="#xptr">[XPTR]</a>.</p>
<div class="example">

<h5>Example: Sample <code>href</code> Attribute Declarations</h5>
<p>Following is a non-normative attribute-list declaration for <code>href</code>
on an element intended to be <code>simple</code>-type.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ATTLIST simplelink
  xlink:href      CDATA           #REQUIRED
  ...&gt;</pre></td></tr></table>
</div></div>
<div class="div2">

<h3><a name="link-semantics"></a>5.5 Semantic Attributes (<code>role</code>, <code>arcrole</code>, and <code>title</code>)</h3>
<p>The attributes that describe the meaning of resources within the context
of a link are <code>role</code>, <code>arcrole</code>, and <code>title</code>. The <code>role</code>
attribute <a title="Must, May, etc." href="#dt-must">may</a> be used on <code>extended</code>-, <code>simple</code>-, <code>locator-</code>,
and <code>resource</code>-type elements. The <code>arcrole</code> attribute <a title="Must, May, etc." href="#dt-must">may</a> be used on <code>arc</code>- and <code>simple</code>-type
elements. The <code>title</code> attribute <a title="Must, May, etc." href="#dt-must">may</a>
be used on all of these types of elements.</p>
<p>The value of the <code>role</code> or <code>arcrole</code> attribute <a title="Must, May, etc." href="#dt-must">must</a> be a URI reference as defined in <a href="#rfc2396">[IETF RFC 2396]</a>,
except that if the URI scheme used is allowed to have absolute and relative
forms, the URI portion <a title="Must, May, etc." href="#dt-must">must</a> be absolute.
The URI reference identifies some resource that describes the intended property.
When no value is supplied, no particular role value is to be inferred. Disallowed
URI reference characters in these attribute values <a title="Must, May, etc." href="#dt-must">must</a>
be specially encoded as described in <a href="#link-locators"><b>5.4 Locator Attribute (href)</b></a>.</p>
<p>The <code>title</code> attribute is used to describe the meaning of a link
or resource in a human-readable fashion, along the same lines as the <code>role</code>
or <code>arcrole</code> attribute. (However, see also <a href="#title-element"><b>5.1.4 Titles for Extended Links, Locators, and Arcs (title-Type Element)</b></a>.)
A value is optional; if a value is supplied, it <a title="Must, May, etc." href="#dt-must">should</a>
contain a string that describes the resource. The use of this information
is highly dependent on the type of processing being done. It <a title="Must, May, etc." href="#dt-must">may</a>
be used, for example, to make titles available to applications used by visually
impaired users, or to create a table of links, or to present help text that
appears when a user lets a mouse pointer hover over a starting resource.</p>
<div class="example">

<h5>Example: Sample <code>role</code> and <code>title</code> Attribute Declarations</h5>
<p>Following is a non-normative attribute-list declaration for <code>role</code>
and <code>title</code> on an element intended to be <code>simple</code>-type.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ATTLIST simplelink
  ...
  xlink:role      CDATA           #IMPLIED
  xlink:title     CDATA           #IMPLIED
  ...&gt;</pre></td></tr></table>
<p>Following is a non-normative attribute-list declaration for <code>arcrole</code>
and <code>title</code> on an element intended to be <code>arc</code>-type.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ATTLIST go
  ...
  xlink:arcrole   CDATA           #IMPLIED
  xlink:title     CDATA           #IMPLIED
  ...&gt;</pre></td></tr></table>
</div></div>
<div class="div2">

<h3><a name="link-behaviors"></a>5.6 Behavior Attributes (<code>show</code> and <code>actuate</code>) </h3>
<p>The behavior attributes are <code>show</code> and <code>actuate</code>. They <a title="Must, May, etc." href="#dt-must">may</a> be used on the <code>simple</code>- and <code>arc</code>-type
elements. When used on a <code>simple</code>-type element, they signal behavior
intentions for traversal to that link's single remote ending resource. When
they are used on an <code>arc</code>-type element, they signal behavior intentions
for traversal to whatever ending resources (local or remote) are specified
by that arc.</p>
<p>The <code>show</code> and <code>actuate</code> attributes are not required.
When they are used, conforming XLink applications <a title="Must, May, etc." href="#dt-must">should</a>
give them the treatment specified in this section. There is no hard requirement
("must") for this treatment because what makes sense for an interactive
application, such as a browser, is unlikely to make sense for a noninteractive
application, such as a robot. However, all applications <a title="Must, May, etc." href="#dt-must">should</a>
take into account the full implications of ignoring the specified behavior
before choosing a different course.</p>
<div class="example">

<h5>Example: Sample <code>show</code> and <code>actuate</code> Attribute Declarations</h5>
<p>Following is a non-normative attribute-list declaration for <code>show</code>
and <code>actuate</code> on an element intended to be <code>simple</code>-type.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ATTLIST simplelink
  xlink:type      (simple)        #FIXED "simple"
  ...
  xlink:show      (new
                  |replace
                  |embed
                  |other
                  |none)          #IMPLIED
  xlink:actuate   (onLoad
                  |onRequest
                  |other
                  |none)          #IMPLIED
  ...&gt;</pre></td></tr></table>
</div>
<p>Applications encountering <code>arc</code>-type elements in linkbase lists <a title="Must, May, etc." href="#dt-must">must</a> treat the behavior attributes as if they were
specified as <code>show="none"</code> and <code>actuate="onLoad"</code>, even
if other values were specified.</p>
<div class="div3">

<h4><a name="show-att"></a>5.6.1 <code>show</code> Attribute</h4>
<p>The <code>show</code> attribute is used to communicate the desired presentation
of the ending resource on traversal from the starting resource.</p>
<div class="constraint">
<p class="prefix"><a name="cn-show-value"></a><b>Constraint: show Value</b></p>
<p>If a value is supplied for a <code>show</code> attribute, it <a title="Must, May, etc." href="#dt-must">must</a>
be one of the values "new", "replace", "embed", "other",
and "none".</p>
</div>
<p>Conforming XLink applications <a title="Must, May, etc." href="#dt-must">should</a> apply
the following treatment for <code>show</code> values:</p>
<dl>
<dt class="label">"new"</dt>
<dd>
<p>An application traversing to the ending resource <a title="Must, May, etc." href="#dt-must">should</a>
load it in a new window, frame, pane, or other relevant presentation context.
This is similar to the effect achieved by the following HTML fragment:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;A HREF="http://www.example.org" target="_blank"&gt;...&lt;/A&gt;</pre></td></tr></table>
</dd>
<dt class="label">"replace"</dt>
<dd>
<p>An application traversing to the ending resource <a title="Must, May, etc." href="#dt-must">should</a>
load the resource in the same window, frame, pane, or other relevant presentation
context in which the starting resource was loaded. This is similar to the
effect achieved by the following HTML fragment:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;A HREF="http://www.example.org" target="_self"&gt;...&lt;/A&gt;</pre></td></tr></table>
</dd>
<dt class="label">"embed"</dt>
<dd>
<p>An application traversing to the ending resource <a title="Must, May, etc." href="#dt-must">should</a>
load its presentation in place of the presentation of the starting resource.
This is similar to the effect achieved by the following HTML fragment:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;IMG SRC="http://www.example.org/smiley.gif" ALT=":-)"&gt;</pre></td></tr></table>
<p>The presentation of the starting resource typically does not consist of
an entire document; it would be the entire document only when the root element
of the document is a simple link. Thus, embedding typically has an effect
distinct from replacing.</p>
<p>Just as for the HTML <code>IMG</code> element, embedding affects only the presentation
of the relevant resources; it does not dictate permanent transformation of
the starting resource. Put another way, when an embedded XLink is processed,
the result of styling the ending resource of the link is merged into the result
of styling the resource into which it is embedded. By contrast, when a construct
such as an XInclude element <a href="#xinclude">[XInclude]</a> is resolved, the original
XML is actually transformed to include the referenced content.</p>
<p>The behavior of conforming XLink applications when embedding XML-based
(<a href="#rfc2376">[IETF RFC 2376]</a> or <a href="#draft-xmlmediatypes">[IETF I-D XMT]</a>) ending resources
is not defined in this version of this specification.</p>
<p>The presentation of embedded resources is application dependent.</p>
</dd>
<dt class="label">"other"</dt>
<dd>
<p>The behavior of an application traversing to the ending resource is unconstrained
by this specification. The application <a title="Must, May, etc." href="#dt-must">should</a>
look for other markup present in the link to determine the appropriate behavior.</p>
</dd>
<dt class="label">"none"</dt>
<dd>
<p>The behavior of an application traversing to the ending resource is unconstrained
by this specification. No other markup is present to help the application
determine the appropriate behavior.</p>
</dd>
</dl>
<p>If the starting or ending resource consists of multiple non-contiguous
locations, such as a series of string ranges in various locations in the resource,
then application behavior is unconstrained. (See <a href="#xptr">[XPTR]</a> for
more information about selecting portions of XML documents.)</p>
<div class="note"><p class="prefix"><b>Note:</b></p>
<p>Some possibilities for application behavior with non-contiguous ending
resources might include highlighting of each location, producing a dialog
box that allows the reader to choose among the locations as if there were
separate arcs leading to each one, concatenating the content of all the locations
for presentation, and so on. Application behavior with non-contiguous starting
resources might include concatenation and rendering as a single unit, or creating
one arc emanating from each contiguous portion.</p>
</div>
</div>
<div class="div3">

<h4><a name="actuate-att"></a>5.6.2 <code>actuate</code> Attribute</h4>
<p>The <code>actuate</code> attribute is used to communicate the desired timing
of traversal from the starting resource to the ending resource..</p>
<div class="constraint">
<p class="prefix"><a name="cn-actuate-value"></a><b>Constraint: actuate Value</b></p>
<p>If a value is supplied for an <code>actuate</code> attribute, it <a title="Must, May, etc." href="#dt-must">must</a> be be one of the values "onLoad", "onRequest", "other",
and "none".</p>
</div>
<p>Conforming XLink applications <a title="Must, May, etc." href="#dt-must">should</a> apply
the following treatment for <code>actuate</code> values:</p>
<dl>
<dt class="label">"onLoad"</dt>
<dd>
<p>An application <a title="Must, May, etc." href="#dt-must">should</a> traverse to the
ending resource immediately on loading the starting resource. This is similar
to the effect typically achieved by the following HTML fragment, when the
user agent is configured to display images:</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;IMG SRC="http://www.example.org/smiley.gif" ALT=":-)"&gt;</pre></td></tr></table>
<p>If a single resource contains multiple arcs whose behavior is set to <code>show="replace"
actuate="onLoad"</code>, application behavior is unconstrained by XLink.</p>
</dd>
<dt class="label">"onRequest"</dt>
<dd>
<p>An application <a title="Must, May, etc." href="#dt-must">should</a> traverse from the
starting resource to the ending resource only on a post-loading event triggered
for the purpose of traversal. An example of such an event might be when a
user clicks on the presentation of the starting resource, or a software module
finishes a countdown that precedes a redirect.</p>
</dd>
<dt class="label">"other"</dt>
<dd>
<p>The behavior of an application traversing to the ending resource is unconstrained
by this specification. The application <a title="Must, May, etc." href="#dt-must">should</a>
look for other markup present in the link to determine the appropriate behavior.</p>
</dd>
<dt class="label">"none"</dt>
<dd>
<p>The behavior of an application traversing to the ending resource is unconstrained
by this specification. No other markup is present to help the application
determine the appropriate behavior.</p>
</dd>
</dl>
</div>
</div>
<div class="div2">

<h3><a name="traversal-atts"></a>5.7 Traversal Attributes (<code>label</code>, <code>from</code>, and <code>to</code>)</h3>
<p>The traversal attributes are <code>label</code>, <code>from</code>, and <code>to</code>.
The <code>label</code> attribute <a title="Must, May, etc." href="#dt-must">may</a> be used
on the <code>resource</code>- and <code>locator</code>-type elements. The <code>from</code>
and <code>to</code> attributes <a title="Must, May, etc." href="#dt-must">may</a> be used
on the <code>arc</code>-type element.</p>
<div class="constraint">
<p class="prefix"><a name="cn-fromto-values"></a><b>Constraint: <code>label</code>, <code>from</code>, and <code>to</code> Values</b></p>
<p>The value of a <code>label</code>, <code>from</code>, or <code>to</code> attribute
must be an <a href="http://www.w3.org/TR/REC-xml-names/#NT-NCName">NCName</a>.
If a value is supplied for a <code>from</code> or <code>to</code> attribute, it <a title="Must, May, etc." href="#dt-must">must</a> correspond to the same value for some <code>label</code>
attribute on a <code>locator</code>- or <code>resource</code>-type element that appears
as a direct child inside the same <code>extended</code>-type element as does the <code>arc</code>-type
element. </p>
</div>
</div>
</div>
</div><div class="back">
<div class="div1">

<h2><a name="references"></a>A References</h2>
<div class="div2">

<h3><a name="N3954"></a>A.1 Normative References</h3>
<dl>
<dt class="label"><a name="draft-xmlmediatypes"></a>IETF I-D XMT</dt><dd><cite>XML Media Types</cite>. Makoto, M., St. Laurent, S. and D. Kohn, editors.
Internet Engineering Task Force, 2001.  (See <a href="http://www.ietf.org/rfc/rfc3023.txt">http://www.ietf.org/rfc/rfc3023.txt</a>.)</dd>
<dt class="label"><a name="rfc2396"></a>IETF RFC 2396</dt><dd>IETF
(Internet Engineering Task Force). <cite>RFC 2396: Uniform Resource Identifiers</cite>.
1995.  (See <a href="http://www.ietf.org/rfc/rfc2396.txt">http://www.ietf.org/rfc/rfc2396.txt</a>.)</dd>
<dt class="label"><a name="rfc2279"></a>IETF RFC 2279</dt><dd><cite>RFC
2279: UTF-8, a transformation format of ISO 10646</cite>. Internet Engineering
Task Force, 1998.  (See <a href="http://www.ietf.org/rfc/rfc2279.txt">http://www.ietf.org/rfc/rfc2279.txt</a>.)</dd>
<dt class="label"><a name="rfc2376"></a>IETF RFC 2376</dt><dd><cite>RFC
2376: XML Media Types</cite>. Internet Engineering Task Force, 1998.  (See <a href="http://www.ietf.org/rfc/rfc2376.txt">http://www.ietf.org/rfc/rfc2376.txt</a>.)</dd>
<dt class="label"><a name="rfc2732"></a>IETF RFC 2732</dt><dd><cite>RFC
2732: Format for Literal IPv6 Addresses in URL's</cite>. Internet Engineering
Task Force, 1999.  (See <a href="http://www.ietf.org/rfc/rfc2732.txt">http://www.ietf.org/rfc/rfc2732.txt</a>.)</dd>
<dt class="label"><a name="XML"></a>XML</dt><dd>Tim Bray, Jean
Paoli, C.M. Sperberg-McQueen, and Eve Maler, editors. <em>Extensible Markup
Language (XML) 1.0 (Second Edition).</em> World Wide Web Consortium, 2000.  (See <a href="http://www.w3.org/TR/2000/REC-xml-20001006">http://www.w3.org/TR/2000/REC-xml-20001006</a>.)</dd>
<dt class="label"><a name="rfc2119"></a>IETF RFC 2119</dt><dd>S.
Bradner, editor. <cite>Key words for use in RFCs to Indicate Requirement
Levels</cite>. March 1997.  (See <a href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a>.)</dd>
<dt class="label"><a name="xbase"></a>XML Base</dt><dd>Jonathan Marsh, editor. <cite>XML Base (XBase)</cite>.
World Wide Web Consortium, 1999.  (See <a href="http://www.w3.org/TR/2001/REC-xmlbase-20010627/">http://www.w3.org/TR/2001/REC-xmlbase-20010627/</a>.)</dd>
<dt class="label"><a name="xname"></a>XML Names</dt><dd>Tim Bray, Dave Hollander, and Andrew Layman, editors. <cite>Namespaces
in XML</cite>. World Wide
Web Consortium, 1999.  (See <a href="http://www.w3.org/TR/1999/REC-xml-names-19990114/">http://www.w3.org/TR/1999/REC-xml-names-19990114/</a>.)</dd>
</dl></div>
<div class="div2">

<h3><a name="N4107"></a>A.2 Non-Normative References</h3>
<dl>
<dt class="label"><a name="chum"></a>CHUM</dt><dd>Steven J. DeRose and David G. Durand. 1995. "The
TEI Hypertext Guidelines." In <cite>Computing and the Humanities</cite>
29(3). Reprinted in <cite>Text Encoding Initiative: Background and Context</cite>,
ed. Nancy Ide and Jean Ronis, ISBN 0-7923-3704-2.</dd>
<dt class="label"><a name="dexter"></a>Dexter</dt><dd>Halasz, Frank. 1994. "The Dexter Hypertext
Reference Model." In Communications of the Association for Computing
Machinery 37 (2), February 1994: 30-39.</dd>
<dt class="label"><a name="fress"></a>FRESS</dt><dd>Steven J. DeRose and Andries van Dam. 1999. "Document
structure in the FRESS Hypertext System." Markup Languages 1 (1) Winter.
Cambridge: MIT Press: 7-32. (See also <a href=" http://www.stg.brown.edu/~sjd/fress.html">
http://www.stg.brown.edu/~sjd/fress.html</a> for more information.)</dd>
<dt class="label"><a name="html"></a>HTML</dt><dd><cite>HTML
4.01 Specification</cite>. World Wide Web Consortium, 1999.  (See <a href="http://www.w3.org/TR/1999/REC-html401-19991224/">http://www.w3.org/TR/1999/REC-html401-19991224/</a>.)</dd>
<dt class="label"><a name="intermedia"></a>Intermedia</dt><dd>Yankelovich, Nicole, Bernard J. Haan,
Norman K. Meyrowitz, and Steven M. Drucker. 1988. "Intermedia: The Concept
and the Construction of a Seamless Information Environment." IEEE Computer
21 (January, 1988): 81-96.</dd>
<dt class="label"><a name="iso10744"></a>ISO/IEC 10744</dt><dd>ISO (International Organization for Standardization). <cite>ISO/IEC
10744-1992 (E). Information technology-Hypermedia/Time-based Structuring Language
(HyTime).</cite> [Geneva]: International Organization for Standardization,
1992. <cite>Extended Facilities Annex.</cite> [Geneva]: International
Organization for Standardization, 1996.  (See <a href="http://www.y12.doe.gov/sgml/wg8/document/1920.htm">http://www.y12.doe.gov/sgml/wg8/document/1920.htm</a>.)</dd>
<dt class="label"><a name="microcosm"></a>MicroCosm</dt><dd>Hall, Wendy, Hugh Davis, and Gerard Hutchings.
1996. <cite>Rethinking Hypermedia: The Microcosm Approach.</cite>
Boston: Kluwer Academic Publishers. ISBN 0-7923-9679-0.</dd>
<dt class="label"><a name="ohs"></a>OHS</dt><dd>van Ossenbruggen, Jacco, Anton Eliëns and Lloyd Rutledge. "The
Role of XML in Open Hypermedia Systems." Position paper for the 4th
Workshop on Open Hypermedia Systems, ACM Hypertext '98.  (See <a href="http://aue.auc.dk/~kock/OHS-HT98/Papers/ossenbruggen.html">http://aue.auc.dk/~kock/OHS-HT98/Papers/ossenbruggen.html</a>.)</dd>
<dt class="label"><a name="rdf"></a>RDF</dt><dd>Ora Lassila and Ralph Swick, editors. <cite>Resource Description
Framework (RDF) Model and Syntax Specification</cite>. World Wide Web
Consortium, 1999.  (See <a href="http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/">http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/</a>.)</dd>
<dt class="label"><a name="tei"></a>TEI</dt><dd>C. M. Sperberg-McQueen and Lou Burnard, editors.<cite>Guidelines
for Electronic Text Encoding and Interchange</cite>. Association for Computers
and the Humanities (ACH), Association for Computational Linguistics (ACL),
and Association for Literary and Linguistic Computing (ALLC). Chicago, Oxford:
Text Encoding Initiative, 1994.</dd>
<dt class="label"><a name="infoset"></a>XIS</dt><dd>John Cowan
and Richard Tobin, editors. <cite>XML Information
Set</cite>. World Wide Web Consortium, 2001.  (See <a href="http://www.w3.org/TR/2001/CR-xml-infoset-20010514/">http://www.w3.org/TR/2001/CR-xml-infoset-20010514/</a>.)</dd>
<dt class="label"><a name="xlink2rdf"></a>XLinkToRDF</dt><dd>Ron Daniel, editor. <cite>Harvesting RDF Statements from
XLinks</cite>. World Wide Web Consortium, 2000.  (See <a href="http://www.w3.org/TR/2000/NOTE-xlink2rdf-20000929/">http://www.w3.org/TR/2000/NOTE-xlink2rdf-20000929/</a>.)</dd>
<dt class="label"><a name="xlink-naming"></a>XLinkNaming</dt><dd>Eve Maler, Daniel Veillard and Henry S. Thompson, editors. <cite>XLink
Markup Name Control</cite>. World Wide Web Consortium, 2000.  (See <a href="http://www.w3.org/TR/2000/NOTE-xlink-naming-20001220/">http://www.w3.org/TR/2000/NOTE-xlink-naming-20001220/</a>.)</dd>
<dt class="label"><a name="xinclude"></a>XInclude</dt><dd>Jonathan Marsh and David Orchard, editors. <cite>XML Inclusions
(XInclude) Version 1.0</cite>. World Wide Web Consortium, 2000.  (See <a href="http://www.w3.org/TR/2001/WD-xinclude-20010516/">http://www.w3.org/TR/2001/WD-xinclude-20010516/</a>.)</dd>
<dt class="label"><a name="xlreq"></a>XLREQ</dt><dd>Steven DeRose, editor. <cite>XML XLink Requirements Version
1.0</cite>.World Wide Web Consortium, 1999.  (See <a href="http://www.w3.org/TR/1999/NOTE-xlink-req-19990224/">http://www.w3.org/TR/1999/NOTE-xlink-req-19990224/</a>.)</dd>
<dt class="label"><a name="xldp"></a>XLDP</dt><dd>Eve
Maler and Steve DeRose, editors. <cite>XML Linking Language (XLink) Design
Principles</cite>.World Wide Web Consortium, 1998.  (See <a href="http://www.w3.org/TR/1998/NOTE-xlink-principles-19980303">http://www.w3.org/TR/1998/NOTE-xlink-principles-19980303</a>.)</dd>
<dt class="label"><a name="xptr"></a>XPTR</dt><dd>Ron Daniel, Steve
DeRose, and Eve Maler, editors.<cite> XML Pointer Language (XPointer)
V1.0</cite>.World Wide Web Consortium, 1998.  (See <a href="http://www.w3.org/TR/2001/WD-xptr-20010108/">http://www.w3.org/TR/2001/WD-xptr-20010108/</a>.)</dd>
</dl></div>
</div>
<div class="div1">

<h2><a name="sample-dtd-appx"></a>B Sample DTD (Non-Normative)</h2>
<p>The following DTD makes invalid (for purposes of argument) all XLink constructs
for which this specification does not specify behavior. It is provided only
as a convenience for application developers; it has no normative status.</p>
<p>The following assumptions hold for this DTD:</p>
<ul>
<li><p>Only constructs that have XLink-defined meaning are allowed.</p>
</li>
<li><p>No "foreign" vocabularies are mixed in, since DTDs do
not work well with namespaces.</p></li>
<li><p>The use of <b>ANY</b> means there is typically content provided
in the element that is used by XLink in some way.</p></li>
<li><p>The use of the <code>(title*)</code> construct means that any non-title
content provided has no XLink-defined use.</p></li>
<li><p>Elements are named after the XLink element types they represent.</p>
</li>
</ul>
<p>Other assumptions and conditions appear as comments in the DTD.</p>
<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>&lt;!ELEMENT simple ANY&gt;
&lt;!ATTLIST simple
   xlink:type      (simple)        #FIXED "simple"
   xlink:href      CDATA           #IMPLIED
   xlink:role      CDATA           #IMPLIED
   xlink:arcrole   CDATA           #IMPLIED
   xlink:title     CDATA           #IMPLIED
   xlink:show      (new
                   |replace
                   |embed
                   |other
                   |none)          #IMPLIED
   xlink:actuate   (onLoad
                   |onRequest
                   |other
                   |none)          #IMPLIED&gt;

&lt;!ELEMENT extended ((title|resource|locator|arc)*)&gt;
&lt;!ATTLIST extended
   xmlns:xlink     CDATA           #FIXED "http://www.w3.org/1999/xlink"
   xlink:type      (extended)      #FIXED "extended"
   xlink:role      CDATA           #IMPLIED
   xlink:title     CDATA           #IMPLIED&gt;

&lt;!ELEMENT title ANY&gt;
&lt;!-- xml:lang is not required, but provides much of the motivation
      for title elements in addition to attributes, and so is provided
      here for convenience --&gt;
&lt;!ATTLIST title
   xlink:type      (title)         #FIXED "title"
   xml:lang        CDATA           #IMPLIED&gt;

&lt;!ELEMENT resource ANY&gt;
&lt;!ATTLIST resource
   xlink:type      (resource)      #FIXED "resource"
   xlink:role      CDATA           #IMPLIED
   xlink:title     CDATA           #IMPLIED
   xlink:label     NMTOKEN         #IMPLIED&gt;

&lt;!ELEMENT locator (title*)&gt;
&lt;!-- label is not required, but locators have no particular XLink
     function if they are not labeled --&gt;
&lt;!ATTLIST locator
   xlink:type      (locator)       #FIXED "locator"
   xlink:href      CDATA           #REQUIRED
   xlink:role      CDATA           #IMPLIED
   xlink:title     CDATA           #IMPLIED
   xlink:label     NMTOKEN         #IMPLIED&gt;

&lt;!ELEMENT arc (title*)&gt;
&lt;!-- from and to have default behavior when values are missing --&gt;
&lt;!ATTLIST arc
   xlink:type      (arc)           #FIXED "arc"
   xlink:arcrole   CDATA           #IMPLIED
   xlink:title     CDATA           #IMPLIED
   xlink:show      (new
                   |replace
                   |embed
                   |other
                   |none)          #IMPLIED
   xlink:actuate   (onLoad
                   |onRequest
                   |other
                   |none)          #IMPLIED
   xlink:from      NMTOKEN         #IMPLIED
   xlink:to        NMTOKEN         #IMPLIED&gt;
</pre></td></tr></table>
</div>
<div class="div1">

<h2><a name="acknowledgements"></a>C Working Group Members and Acknowledgments (Non-Normative)</h2>
<p>This specification was produced in the XML Linking Working Group, with
the following members active at the completion of this specification:</p>
<ul>
<li>Peter Chen, LSU, Bootstrap Alliance
</li>
<li>Ron Daniel, Interwoven</li>
<li>Steve DeRose, Brown University Scholarly Technology
Group (<i>XLink co-editor</i>) </li>
<li>David Durand, University of Southhampton,
Dynamic Diagrams</li>
<li>Masatomo Goto, Fujitsu Laboratories
</li>
<li>Paul Grosso, Arbortext</li>
<li>Chris Maden, Lexica</li>
<li>Eve Maler, Sun Microsystems
 (<i>co-chair and XLink co-editor</i>) </li>
<li>Jonathan Marsh, Microsoft</li>
<li>David Orchard, Jamcracker (<i>XLink
co-editor</i>) </li>
<li>Henry S. Thompson, University of
Edinburgh, W3C</li>
<li>Daniel Veillard, W3C staff contact
 (<i>co-chair</i>) </li>
</ul>
<p>The editors wish to acknowledge substantial contributions from Tim Bray,
who previously served as co-editor and co-chair, and Ben Trafford, who previously
served as co-editor. We would also like to acknowledge important contributions
from Gabe Beged-Dov, who wrote the XArc proposal. Finally, we would like to
thank the XML Linking Interest Group and Working Group for their support and
input, and Henry Thompson, for helping chair the group and act as staff
contact for the last few months before this publication.</p>
</div>
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