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  <title>XHTML and RDF</title>
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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><a name="title" id="title"></a>XHTML and RDF</h1>

<h2><a name="w3c-doctype" id="w3c-doctype"></a>Version 14 February 2004</h2>
<dl>
  <dt>This version: </dt>
    <dd>http://www.w3.org/MarkUp/2004/02/xhtml-rdf.html</dd>
  <dt>Editor:</dt>
    <dd>Mark Birbeck, x-port.net Ltd. <a
      href="Mark.Birbeck@x-port.net">&lt;Mark.Birbeck@x-port.net&gt;</a></dd>
</dl>

<p class="copyright"><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>&nbsp;&copy;&nbsp;2004&nbsp;<a
href="http://www.w3.org/"><abbr
title="World Wide Web Consortium">W3C</abbr></a><sup>&reg;</sup> (<a
href="http://www.lcs.mit.edu/"><abbr
title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
href="http://www.ercim.org/"><abbr
title="European Research Consortium for Informatics and Mathematics">ERCIM</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#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>,
<a href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a>, and <a
href="http://www.w3.org/Consortium/Legal/copyright-software">software
licensing</a> rules apply.</p>
</div>
<hr>

<div>
<h2><a name="abstract" id="abstract"></a>Abstract</h2>

<p>The aim of this document is to discuss the relationship between XHTML and
metadata.</p>
</div>

<div>
<h2><a name="status" id="status"></a>Status of this Document</h2>

<p>This is an internal working draft.</p>

<p><em>Last Modified: $Date: 2004/02/27 11:55:44 $</em></p>
</div>

<div class="toc">
<h2><a name="contents" id="contents"></a>Table of Contents</h2>

<p class="toc">1 <a href="#div151383088">Motivation</a><br>
2 <a href="#div151384000">Types of Metadata in XHTML Documents</a><br>
3 <a href="#div154317128">Document Metadata</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;3.1 <a href="#div254317416">Top-level Metadata with
meta and QNames</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;3.2 <a href="#div254358472">Statements About
Top-level Metadata</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;3.3 <a href="#div254363072">Statements About Other
Resources</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;3.4 <a href="#div254370032">String Literals and
URIs</a><br>
4 <a href="#div154379976">Markup Metadata</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;4.1 <a href="#div254386928">What Are We Trying To
Represent?</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;4.2 <a href="#div254333552">Identifying
Resources</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;4.3 <a href="#div254343520">Representing Type</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;4.4 <a href="#div254348288">Representing Value</a><br>
5 <a href="#div154351016">Bibliography</a><br>
</p>
</div>
<hr>

<div class="body">

<div class="div1">
<h2><a name="div151383088" id="div151383088"></a>1 Motivation</h2>

<p>We have two standards running parallel with each other; HTML is the de
facto standard for document markup, accounting for millions of items on the
web. RDF is a standard for expressing metadata, which in turn provides a
foundation for making use of that metadata, such as reasoning about it. Yet
the former is very rarely the subject of the latter; meta information placed
in the HTML family of documents is often encoded in such a way as to make it
difficult to extract by RDF-related parsers. And if it cannot be extracted,
then it cannot be used.</p>

<p>Our intention here is to make more of the information that is contained
within HTML-family documents available to RDF tools, but without putting an
unnecessary burden on authors familiar with HTML, but not with the subtleties
of triples and statements. However, for our discussions on how best to do
this, we do need to be familiar with at least the principles of RDF.</p>

<p>RDF is about statements and triples. There are a number of syntaxes which
can be used to express these triples, such as N3 and RDF/XML. This document
proposes ensuring that the metadata elements already proposed in XHTML 2.0
can be used to glean useful RDF-based information.</p>
</div>

<div class="div1">
<h2><a name="div151384000" id="div151384000"></a>2 Types of Metadata in XHTML
Documents</h2>

<p>There are a number of sets of useful meta information that may be
contained in, or relate to, an XHTML document:</p>
<ul>
  <li><p>There may be information about the document itself; who wrote it and
    when, the document's subject matter, and so on.</p>
  </li>
  <li><p>There may be further information about some of this document
    metadata; not only might we indicate the name of the author, but we might
    also mark-up metadata to indicate their contact information.</p>
  </li>
  <li><p>There may be information about words used in the text; an
    abbreviation may be marked-up with the full text version, or the word
    "yesterday" may be tagged as being a date with a value of
    "2004-01-01".</p>
  </li>
  <li><p>And finally, there may be information about items completely
    unrelated to the document; there may be meta information about a company
    or country.</p>
  </li>
</ul>

<p>To introduce the issues we'll go through some examples of each of these,
using the RDF notion of triples.</p>
</div>

<div class="div1">
<h2><a name="div154317128" id="div154317128"></a>3 Document Metadata</h2>

<div class="div2">
<h3><a name="div254317416" id="div254317416"></a>3.1 Top-level Metadata with
<code>meta</code> and QNames</h3>

<p>Our first scenario involves statements made about the document. This is to
some extent already catered for with the current use of <code>meta</code> and
<code>link</code>. Statements can be made about the document, such as its
publication date, author, revision number, subjects, and so on.</p>

<p>An example of using <code>meta</code> is provided in the latest XHTML 2.0
draft:</p>

<div class="exampleInner">
<pre>&lt;html ... profile="http://www.acme.com/profiles/core"&gt;
        &lt;head&gt;
                &lt;title&gt;How to complete Memorandum cover sheets&lt;/title&gt;
                &lt;meta name="author"&gt;John Doe&lt;/meta&gt;
                &lt;meta name="copyright"&gt;&amp;copy; 1997 Acme Corp.&lt;/meta&gt;
                &lt;meta name="keywords"&gt;corporate,guidelines,cataloging&lt;/meta&gt;
                &lt;meta name="date"&gt;1994-11-06T08:49:37+00:00&lt;/meta&gt;
        &lt;/head&gt;
        ...
                        </pre>
</div>

<p>Whilst this provides useful metadata about the document, the lack of
qualification of the value in the <code>name</code> attribute means that in
terms of RDF statements, it is weak. The technique usually used in previous
versions of HTML is to provide a prefix on the property name, or to use the
<code>scheme</code> attribute. Adopting this same technique in XHTML 2.0, our
markup would now look like this:</p>

<div class="exampleInner">
<pre>&lt;meta name="DC.identifier"&gt;http://www.rfc-editor.org/rfc/rfc3236.txt&lt;/meta&gt;
                        </pre>
</div>

<p>From an RDF standpoint this doesn't help. The statement that the author
intends - expressed as RDF triples - would be:</p>

<div class="exampleInner">
<pre>&lt;&gt; &lt;http://purl.org/dc/elements/1.1/identifier&gt; &lt;http://www.rfc-editor.org/rfc/rfc3236.txt&gt; .
                        </pre>
</div>

<table border="1" summary="Editorial note">
  <tbody>
    <tr>
      <td align="left" valign="top" width="50%"><b>Editorial note</b></td>
      <td align="right" valign="top" width="50%">&nbsp;</td>
    </tr>
    <tr>
      <td colspan="2" align="left" valign="top">All RDF statements are
        represented in this document using N3, since it is more compact than
        RDF/XML. Most of the notation I will use is covered in <a
        href="#N3-PRIMER">[N3-PRIMER]</a>.</td>
    </tr>
  </tbody>
</table>

<p>An easy way to resolve this would be to allow the author to use QNames
inside <code>name</code>:</p>

<div class="exampleInner">
<pre>&lt;html xmlns:dc="http://purl.org/dc/elements/1.1/"&gt;
        &lt;head&gt;
                &lt;title&gt;How to complete Memorandum cover sheets&lt;/title&gt;
                &lt;meta name="dc:creator"&gt;John Doe&lt;/meta&gt;
                &lt;meta name="dc:rights"&gt;&amp;copy; 1997 Acme Corp.&lt;/meta&gt;
                &lt;meta name="dc:subject"&gt;corporate,guidelines,cataloging&lt;/meta&gt;
                &lt;meta name="dc:date"&gt;1994-11-06T08:49:37+00:00&lt;/meta&gt;
        &lt;/head&gt;
        ...
                        </pre>
</div>

<p>However, for reasons that will become clearer later, I would suggest that
we drop the use of <code>name</code> and call it <code>property</code>:</p>

<div class="exampleInner">
<pre>&lt;html xmlns:dc="http://purl.org/dc/elements/1.1/"&gt;
        &lt;head&gt;
                &lt;title&gt;How to complete Memorandum cover sheets&lt;/title&gt;
                &lt;meta property="dc:creator"&gt;John Doe&lt;/meta&gt;
                &lt;meta property="dc:rights"&gt;&amp;copy; 1997 Acme Corp.&lt;/meta&gt;
                &lt;meta property="dc:subject"&gt;corporate,guidelines,cataloging&lt;/meta&gt;
                &lt;meta property="dc:date"&gt;1994-11-06T08:49:37+00:00&lt;/meta&gt;
        &lt;/head&gt;
        ...
                        </pre>
</div>

<p>This would create the following statements:</p>

<div class="exampleInner">
<pre>@prefix dc:  &lt;http://purl.org/dc/elements/1.1/&gt; .
&lt;&gt; dc:creator "John Doe" ;
   dc:rights "&amp;copy; 1997 Acme Corp." ;
   dc:subject "corporate,guidelines,cataloging" ;
   dc:date "1994-11-06T08:49:37+00:00" .
                        </pre>
</div>

<table border="1" summary="Editorial note">
  <tbody>
    <tr>
      <td align="left" valign="top" width="50%"><b>Editorial note</b></td>
      <td align="right" valign="top" width="50%">&nbsp;</td>
    </tr>
    <tr>
      <td colspan="2" align="left" valign="top">This would mean that in the
        previous example, non-prefixed names would be regarded as being
        properties from the source document - for example <code>&lt;&gt;
        :author "John Doe" .</code> - which I would suggest is correct. The
        consequence is that no matter how consistently a name is applied
        across a web site, it never conveys anything other than a 'local'
        concept.</td>
    </tr>
  </tbody>
</table>
</div>

<div class="div2">
<h3><a name="div254358472" id="div254358472"></a>3.2 Statements About
Top-level Metadata</h3>

<p>Our second scenario concerns statements made about the metadata that we
have just created to describe the document. We might want to add further
information about the author or publisher, for example.</p>

<p>The simplest way to do this would be to place the additional statements in
an external RDF/XML document, and make reference to them. For example, the
external reference could, under the current spec, be expressed like this:</p>

<div class="exampleInner">
<pre>&lt;link rel="meta" href="JohnDoe.rdf" /&gt;
                                </pre>
</div>

<p>and the metadata in the document being linked to, might look like this:</p>

<div class="exampleInner">
<pre>&lt;rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:con="http://www.w3.org/2000/10/swap/pim/contact#"
&gt;
        &lt;con:Person rdf:about="http://www.example.org/People/JohnDoe"&gt;
                &lt;con:fullName&gt;John Doe&lt;/con:fullName&gt;
                &lt;con:mailbox rdf:resource="mailto:JohnDoe@example.org"/&gt;
        &lt;/con:Person&gt;
&lt;/rdf:RDF&gt;
                                </pre>
</div>

<p>Whilst this makes the metadata available to an RDF/XML parser, this does
imply that if this information were to be usable <em>within</em> the XHTML
2.0 browser, then it would need to incorporate an RDF/XML parser. Whilst this
is certainly a desirable goal in the long term, it is probably an impractical
requirement in the short. Anyway, there are other ways of structuring the
data that may give the XHTML 2.0 browser access to the meta information,
without requiring it to incorporate an RDF/XML parser.</p>

<p>Unlike previous HTML mark-up, XHTML 2.0 allows <code>meta</code> to be
nested. The effect of this in RDF terms, is to create an anonymous resource
inside the containing <code>meta</code> element, and then for nested
<code>meta</code> elements to be further statements about this anonymous
resource. This technique would therefore allow the previous information about
our document's author to be used by the browser, or indeed the author,
without requiring a full-fledged RDF parser. The structure might look
something like this (some statements from the external document are not
reflected):</p>

<div class="exampleInner">
<pre>&lt;html xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:con="http://www.w3.org/2000/10/swap/pim/contact#"&gt;
        &lt;head&gt;
                &lt;title&gt;How to complete Memorandum cover sheets&lt;/title&gt;
                &lt;meta property="dc:creator"&gt;
                        &lt;meta property="con:fullName"&gt;John Doe&lt;/meta&gt;
                &lt;/meta&gt;
                &lt;meta property="dc:rights"&gt;&amp;copy; 1997 Acme Corp.&lt;/meta&gt;
                &lt;meta property="dc:subject"&gt;corporate,guidelines,cataloging&lt;/meta&gt;
                &lt;meta property="dc:date"&gt;1994-11-06T08:49:37+00:00&lt;/meta&gt;
        &lt;/head&gt;
        ...
                        </pre>
</div>

<p>This would be read in prose as "the document's author is called 'John
Doe'", and in terms of statements that "this document has a dc:creator
property, which is an anonymous object, which in turn has a con:fullName
property, which is 'John Doe'". Note that the implication here is that any
non-contained <code>meta</code> element is referring to "this document". In
N3 notation it would be:</p>

<div class="exampleInner">
<pre>&lt;&gt; dc:creator [ con:fullName "John Doe" ] .
                                </pre>
</div>
</div>

<div class="div2">
<h3><a name="div254363072" id="div254363072"></a>3.3 Statements About Other
Resources</h3>

<p>Sometimes we need to be able to make statements about resources outside of
our document. For example, we may know that the resource identified by
<code>http://people.com/TonyBlair</code> has a <code>con:fullName</code> of
"Tony Blair". Currently the only choices we have are either to embed RDF/XML
inside our XHTML document, or to link to an external RDF file, in the manner
that we saw above.</p>

<p>Since neither of these approches is suitable, I would suggest that we add
a new attribute <code>about</code>that can appear on the <code>meta</code>
element. Our new markup might look like this:</p>

<div class="exampleInner">
<pre>&lt;html ...&gt;
        &lt;head&gt;
                &lt;title&gt;How to complete Memorandum cover sheets&lt;/title&gt;
                &lt;meta property="dc:creator"&gt;
                        &lt;meta property="con:fullName"&gt;John Doe&lt;/meta&gt;
                &lt;/meta&gt;
                &lt;meta about="p:TonyBlair" property="con:fullName"&gt;Tony Blair&lt;/meta&gt;
        &lt;/head&gt;
        ...
                                </pre>
</div>

<p>This time our triples contain one statement about our document (and a
statement about that statement), followed by one about some external resource
over which we have no control:</p>

<div class="exampleInner">
<pre>&lt;&gt; dc:creator [ con:fullName "John Doe" ] .
p:TonyBlair con:fullName "Tony Blair" .
                                </pre>
</div>

<p>Another consequence of adding <code>about</code> is that we can now make
statments about other parts of our document. For example, if we know that one
part of the document is actually attributable to someone else, we could use
the following markup:</p>

<div class="exampleInner">
<pre>&lt;html ...&gt;
        &lt;head&gt;
                &lt;title&gt;Some quotes&lt;/title&gt;
                &lt;meta about="#q1"&gt;
                        &lt;meta property="dc:source"&gt;http://www.example.com/tolkien/twotowers.html&lt;/meta&gt;
                &lt;/meta&gt;
        &lt;/head&gt;
        &lt;body&gt;
                &lt;blockquote id="q1"&gt;
                        &lt;p&gt;They went in single file, running like hounds on a strong scent,
                        and an eager light was in their eyes. Nearly due west the broad
                        swath of the marching Orcs tramped its ugly slot; the sweet grass
                        of Rohan had been bruised and blackened as they passed.&lt;/p&gt;
                &lt;/blockquote&gt;
        &lt;/body&gt;
&lt;/html&gt;
                                </pre>
</div>

<table border="1" summary="Editorial note">
  <tbody>
    <tr>
      <td align="left" valign="top" width="50%"><b>Editorial note</b></td>
      <td align="right" valign="top" width="50%">&nbsp;</td>
    </tr>
    <tr>
      <td colspan="2" align="left" valign="top">This is a reworking of the
        example at <a
        href="http://www.w3.org/MarkUp/Group/2003/WD-xhtml2-20031029/mod-structural.html#sec_8.3.">8.3</a>.
        Note, however that we favour the Dublin Core's <code>source</code>
        element over the proposed XHTML 2.0 <code>cite</code> element, which
        was used in the original example.</td>
    </tr>
  </tbody>
</table>

<p>The triple represented is as follows:</p>

<div class="exampleInner">
<pre>&lt;#q1&gt; dc:source "http://www.example.com/tolkien/twotowers.html" .
                                </pre>
</div>
</div>

<div class="div2">
<h3><a name="div254370032" id="div254370032"></a>3.4 String Literals and
URIs</h3>

<p>Although allowing QNames in <code>property</code> and <code>about</code>
on <code>meta</code> gets us a little closer to usable RDF statements, there
is still another problem; RDF allows statements to be made about two types of
objects - resources and string literals - but the syntax that we have built
up so far, only allows the object of a statement to be a string literal. The
consequence is that this:</p>

<div class="exampleInner">
<pre>&lt;meta property="dc:identifier"&gt;http://www.rfc-editor.org/rfc/rfc3236.txt&lt;/meta&gt;
                                </pre>
</div>

<p>produces the following triples:</p>

<div class="exampleInner">
<pre>&lt;&gt; dc:identifier "http://www.rfc-editor.org/rfc/rfc3236.txt" .
                                </pre>
</div>

<p>As you can see, the object of the statement is a string literal, even
though we wanted it to be a resource. The statement we really wanted to make,
was this:</p>

<div class="exampleInner">
<pre>&lt;&gt; dc:identifier &lt;http://www.rfc-editor.org/rfc/rfc3236.txt&gt; .
                                </pre>
</div>

<p>Since XHTML 2.0 brings <code>link</code> with it from HTML to express
relationships between the source document and some other document, then I
would suggest that we simply allow QNames in the <code>rel</code> attribute
on <code>link</code>. Our Dublin Core <code>identifier</code> example would
then be expressed as follows:</p>

<div class="exampleInner">
<pre>&lt;link rel="dc:identifier" href="http://www.rfc-editor.org/rfc/rfc3236.txt" /&gt;
                                </pre>
</div>

<table border="1" summary="Editorial note">
  <tbody>
    <tr>
      <td align="left" valign="top" width="50%"><b>Editorial note</b></td>
      <td align="right" valign="top" width="50%">&nbsp;</td>
    </tr>
    <tr>
      <td colspan="2" align="left" valign="top">To make this consistent, the
        values in <code>rel</code><a
        href="#XHTML-2.0-LINKTYPES">[XHTML-2.0-LINKTYPES]</a> would all need
        to be namespace prefixed, for example <code>xhtml2:next</code> or
        <code>xhtml2:stylesheet</code>. This is no bad thing.</td>
    </tr>
  </tbody>
</table>

<p>In addition we would allow <code>link</code> to appear anywhere that
<code>meta</code> can, since we are saying that the only difference between
them is that one identifies a property of the document and assigns it a
string literal value, whilst the other identifies a property of the document
and assigns it a resource as the value. Our previous example (in which we
indicated the source of a quote), would then become this:</p>

<div class="exampleInner">
<pre>&lt;meta about="#q1"&gt;
        &lt;link rel="dc:source" href="http://www.example.com/tolkien/twotowers.html" /&gt;
&lt;/meta&gt;
                                </pre>
</div>

<p>Note that wheras before the triples produced were these:</p>

<div class="exampleInner">
<pre>&lt;#q1&gt; dc:source "http://www.example.com/tolkien/twotowers.html" .
                                </pre>
</div>

<p>now we correctly have these:</p>

<div class="exampleInner">
<pre>&lt;#q1&gt; dc:source &lt;http://www.example.com/tolkien/twotowers.html&gt; .
                                </pre>
</div>

<p>There is no reason why a number of the attributes for <code>meta</code>
and <code>link</code> cannot appear on the same element. Our example could
therefore be abbreviated to:</p>

<div class="exampleInner">
<pre>&lt;link about="#q1" rel="dc:source" href="http://www.example.com/tolkien/twotowers.html" /&gt;
                                </pre>
</div>
</div>
</div>

<div class="div1">
<h2><a name="div154379976" id="div154379976"></a>4 Markup Metadata</h2>

<p>The next category of metadata concerns the qualification of text that
appears in a document. This whole area is often discussed on the HTML mailing
lists, and is the source of much confusiion. Almost every other day someone
proposes new markup to capture the notion of weight, time, addresses, and so
on. It's therefore worth looking at what the various proposals are trying to
capture.</p>

<p>If we were simply concerned with how a document <em>appeared</em> then we
would not need new markup. For example, if we have a corporate website, and
on the 'contact us' page the address of the company headquarters is
displayed, then the following would be sufficient:</p>

<div class="exampleInner">
<pre>&lt;p class="address"&gt;
   &lt;span class="street"&gt;4 Pear Treet Court&lt;/span&gt;
   &lt;span class="city"&gt;London&lt;/span&gt;
   &lt;span class="country"&gt;United Kingdom&lt;/span&gt;
&lt;/p&gt;</pre>
</div>

<p>We could then add a stylesheet, and have the city shown in bold, red,
loud, or whatever.</p>

<p>However, an automated process that was analysing this 'address' would be
unlikely to derive any 'meaning' from this markup. To 'make sense' of this
information, an indexing engine, content management system, e-commerce
server, or whatever, would need to know in advance what 'street' and 'city'
meant.</p>

<p>Of course, we could achieve this if we all agreed to reserve those words
in the <code>class</code> attribute to mean only 'parts of an address', but
that would then break with people who want to use 'city' as a style class,
with no relation to its semantic meaning. It would also break for people who
want to use 'address' for an IP address. In other words, we would be imposing
a meaning onto those that don't want it. Far better then to try to create
'unique' versions of these identifiers.</p>

<p>What is usually proposed is to create some elements that capture the
notions of <em>addresses</em>. The author's presentational requirements are
not compromised since they are still able to style the new markup, and of
course the elements capture the semantics:</p>

<div class="exampleInner">
<pre>&lt;style&gt;
   city { background-color: teal; }
&lt;/style&gt;
&lt;address&gt;
   &lt;street&gt;4 Pear Treet Court&lt;/street&gt;
   &lt;city&gt;London&lt;/city&gt;
   &lt;country&gt;United Kingdom&lt;/country&gt;
&lt;/address&gt;</pre>
</div>

<p>Whilst the general approach is fine - the use of elements to clearly mark
metadata - the fact that those elements exist in the XHTML namespace causes a
number of problems. The main one is that it is not clear where we should draw
the line when adding new elements to XHTML; as mentioned earlier, on the
<code>www-html</code> list debates frequently run about whether
date/time/quantity/happiness elements should be added to XHTML. All of them
have worthy arguments for their inclusion, but the bigger issue is whether we
should even bother (since where do we stop), and should we instead focus on
providing mechanisms by which elements from other namespaces can be used?</p>

<p>The second problem with these elements appearing in the XHTML namespace
concerns their actual 'meaning'. Since all of these elements exist in the
XHTML 2.0 namespace, then to some metadata processor, what we actually have
is an 'XHTML date' (or an 'XHTML address', or person, or whatever).</p>

<p>One solution to this might appear to be to use the techniques we described
above - <code>meta</code> and <code>link</code>:</p>

<div class="exampleInner">
<pre>&lt;html xmlns:con="http://www.w3.org/2000/10/swap/pim/contact#"&gt;
&lt;head&gt;
    &lt;meta property="con:address"&gt;
        &lt;meta property="con:street"&gt;4 Pear Treet Court&lt;/meta&gt;
        &lt;meta property="con:city"&gt;London&lt;/meta&gt;
        &lt;meta property="con:country"&gt;United Kingdom&lt;/meta&gt;
    &lt;/meta&gt;
&lt;/head&gt;
...
                        </pre>
</div>

<p>The statements represented by this are</p>

<div class="exampleInner">
<pre>&lt;&gt; con:address
  [
      con:street "4 Pear Tree Court" ;
      con:city "London" ;
      con:country "United Kingdom"
  ] .
                        </pre>
</div>

<p>Note however, that this indicates that it is the current document that
'has an address'. This is something that is often overlooked in the
discussions about new markup to represent meta information - what exactly is
it representing? To return to the type of example most often given:</p>

<div class="exampleInner">
<pre>&lt;style&gt;
   city { background-color: teal; }
&lt;/style&gt;
&lt;address&gt;
   &lt;street&gt;4 Pear Treet Court&lt;/street&gt;
   &lt;city&gt;London&lt;/city&gt;
   &lt;country&gt;United Kingdom&lt;/country&gt;
&lt;/address&gt;
                        </pre>
</div>

<p>what is it that is based in London? And more to the point, what exactly
<em>is</em> London. Let's turn to these questions now.</p>

<div class="div2">
<h3><a name="div254386928" id="div254386928"></a>4.1 What Are We Trying To
Represent?</h3>

<p>When we are using <code>meta</code> it is clear that this:</p>

<div class="exampleInner">
<pre>&lt;meta property="dc:creator"&gt;John Doe&lt;/meta&gt;
                                </pre>
</div>

<p>means:</p>

<div class="exampleInner">
<pre>"John Doe" is <em>the creator</em> of <em>this document</em></pre>
</div>

<p>or:</p>

<div class="exampleInner">
<pre>&lt;&gt; dc:creator "John Doe" .
                                </pre>
</div>

<p>As we know, <code>meta</code> has an implied subject of the document that
contains it. But when we say:</p>

<div class="exampleInner">
<pre>&lt;city&gt;London&lt;/city&gt;
                                </pre>
</div>

<p>What is it that 'has' a property of city? We are probably not saying that
the document has a property of city, with a value of 'London'. In fact, the
only thing that we <em>can</em> be saying is that the text string 'London'
has a <em>type</em> of <em>city</em>.</p>

<p>And why might authors want to emphasise this? In the hope that when
someone uses some search engine to find documents about 'the city called
London', they will find this document, and when they search for documents
about Jack London, they won't.</p>

<p>Currently there is no easy way to indicate in a document which of these
two queries you would like your document to repsond to. We could indicate the
subject of a document with <code>meta</code>, by setting
<code>property</code> to "dc:subject":</p>

<div class="exampleInner">
<pre>&lt;meta property="dc:subject"&gt;London&lt;/meta&gt;
                                </pre>
</div>

<p>but there is nothing here to say whether we are writing about Jack London,
or the city.</p>

<p>Solving this problem would also help internationalisation, since currently
the only options to help French users to find this document would be to mark
the document up like this:.</p>

<div class="exampleInner">
<pre>&lt;meta property="dc:subject"&gt;London, Londres&lt;/meta&gt;
                                </pre>
</div>

<p>or this:.</p>

<div class="exampleInner">
<pre>&lt;meta xml:lang="en" property="dc:subject"&gt;London&lt;/meta&gt;
&lt;meta xml:lang="fr" property="dc:subject"&gt;Londres&lt;/meta&gt;
                                </pre>
</div>

<p>neither of which are very practical.</p>

<p>So, we seem to be stuck; if we start at the top of the document, and say
that</p>

<div class="exampleInner">
<pre>"London" is <em>a subject</em> of <em>this document</em></pre>
</div>

<p>we don't know if we mean Jack London, or London Town. But if we start at
the bottom of the document and say that:</p>

<div class="exampleInner">
<pre>"London" is <em>a type</em> of <em>city</em></pre>
</div>

<p>we're none the wiser as to how this piece of information relates to
anything else. Using RDF statements again, we can say that the top-down
approach is represented by:</p>

<div class="exampleInner">
<pre>&lt;&gt; dc:subject "London" .
                                </pre>
</div>

<p>and the bottom-up, by:</p>

<div class="exampleInner">
<pre>[ :city "London" ] .
                                </pre>
</div>

<p>Our task then is to see if we can make the two meet in the middle, and our
first step is to solve the problem of searching for 'London' versus
'Londres'.</p>
</div>

<div class="div2">
<h3><a name="div254333552" id="div254333552"></a>4.2 Identifying
Resources</h3>

<p>To re-cap, the issue is that the type of markup that is often proposed,
such as:</p>

<div class="exampleInner">
<pre>&lt;city&gt;London&lt;/city&gt;
                                </pre>
</div>

<p>is not much use to us, since we do not know what the information relates
to. In this case, which city is it that has a string literal value of
"London"? The key to this is of course to use a unique value based on URIs.
If we had one defined for the city of London, in the UK, such as
<code>http://www.examples.org/city#london</code>, and we were able to attach
this in some way to our text, then we could unambiguously identify what we're
talking about.</p>

<p>I propose therefore, that we add an attribute called
<code>resource</code>, which would allow us to do the following:</p>

<div class="exampleInner">
<pre>&lt;city resource="city:london"&gt;London&lt;/city&gt;
                                </pre>
</div>

<p>We have now uniquely identified that the document we're dealing with has a
reference to the city of London, in the UK, regardless of the actual text
used in the document:</p>

<div class="exampleInner">
<pre>I stayed in the &lt;city resource="city:london"&gt;capital&lt;/city&gt; for a week.
Je voudrais visiter a &lt;city resource="city:london"&gt;Londres&lt;/city&gt;.
                                </pre>
</div>

<p>Now a search engine could search for all documents that contain a
reference to <code>http://www.examples.org/city#london</code>, which would
unambiguously identify documents relating to the city of London, in the UK.
And provided that the search server had a mapping between "Londres" and
<code>http://www.examples.org/city#london</code>, French speakers could also
find this article. (And the indexing server may have deduced the mapping when
crawling documents.)</p>

<p>As we can see with the use of this technique to make documents available
to searches in other languages, it is particularly powerful when the term
being searched for is not present in the document. For example, with this
document:</p>

<div class="exampleInner">
<pre>Tomorrow the Prime Minister is expected to fly to ...
                                </pre>
</div>

<p>Searching for "Tony Blair" would not find this article. However, if a
search server knows how to establish from a user that they are actually
searching for <code>http://people.com/TonyBlair</code>, then we could mark up
our document as follows:</p>

<div class="exampleInner">
<pre>Tomorrow the &lt;span resource="p:TonyBlair"&gt;Prime Minister&lt;/span&gt;
is expected to fly to ...
                                </pre>
</div>

<p>A search for articles relating to Tony Blair would now yield this one,
even though his name is not mentioned. This becomes particularly useful when
individuals and places are identified in a number of ways, and the lexical
description bears no relation to the actual meaning:</p>

<div class="exampleInner">
<pre>Today &lt;span resource="p:DianaSpencer"&gt;Lady Diana&lt;/span&gt; died in a car crash ...
The &lt;span resource="p:DianaSpencer"&gt;Princess of Wales&lt;/span&gt; was killed today ...
                                </pre>
</div>

<p>One final example concerns distinguishing between two items of text that
are the same, but represent something different. For example:</p>

<div class="exampleInner">
<pre>Yesterday in Parliament the &lt;span resource="p:WinstonChurchill"&gt;Prime Minister&lt;/span&gt;
said that we will fight on the beaches ...
Tomorrow the &lt;span resource="p:TonyBlair"&gt;Prime Minister&lt;/span&gt;
is expected to fly to ...</pre>
</div>

<p>In these two examples the string of characters "Prime Minister" is exactly
the same, but they refer to two different individuals. It is now possible to
search for "Winston Churchill" and still find the first of these two
articles, but not the second. Indeed, where we to search for "Prime
Minister", our search server could easily ask us whether we want to search
for the words "Prime Minister", or for articles about the current or a
previous prime minister.</p>

<table border="1" summary="Editorial note">
  <tbody>
    <tr>
      <td align="left" valign="top" width="50%"><b>Editorial note</b></td>
      <td align="right" valign="top" width="50%">&nbsp;</td>
    </tr>
    <tr>
      <td colspan="2" align="left" valign="top">Of course, it is also now
        possible to search for "all articles about British prime ministers
        who served during the second world war", provided that the search
        engine is able to deduce such things. As long as the answer includes
        <code>p:WinstonChurchill</code> then this document can be
      located.</td>
    </tr>
  </tbody>
</table>

<p>One last thing on resources, we still haven't addressed where this
information is attached to. The easiest approach is to say that where any of
our new attributes appear on an element that is not <code>meta</code>, then
there is an implied relationship to the containing resource (or the document)
of "xhtml2:reference". Our previous example therefore results in the
following triples:</p>

<div class="exampleInner">
<pre>&lt;&gt; xh2:reference p:TonyBlair .
                                </pre>
</div>

<p>This means simply that <em>this document</em> contains <em>a
reference</em> to <em>Tony Blair</em>. Note that this is not just a fudge -
we are saying that a search for articles and documents written by, reviewed
by, criticised by, or summarised by Tony Blair, is very different to seeking
out articles and documents that <em>make reference</em> to Tony Blair. This
additional property allows us to make that distinction.</p>
</div>

<div class="div2">
<h3><a name="div254343520" id="div254343520"></a>4.3 Representing Type</h3>

<p>Our second issue was to be able to express the <em>type</em> of our
element. I would propose that <code>property</code> is allowed on any
element, not just <code>meta</code>. For example:</p>

<div class="exampleInner">
<pre>&lt;span property="con:address"&gt;
        &lt;span property="con:street"&gt;4 Pear Tree Court&lt;/span&gt;
        &lt;span property="con:city"&gt;London&lt;/span&gt;
        &lt;span property="con:country"&gt;United Kingdom&lt;/span&gt;
&lt;/span&gt;</pre>
</div>

<p>This now opens up an unlimited supply of descriptors for our text, and so
rather than trying to add elements such as address, weight, time and so on we
can make use of established taxonomies, or devise our own.</p>

<p>Note also that the presence of <code>property</code> creates opportunities
for avoiding repetition. For example, we often find ourselves creating our
metadata from information that also appears in the document itself:</p>

<div class="exampleInner">
<pre>&lt;html xmlns:dc="http://purl.org/dc/elements/1.1/"&gt;
        &lt;head&gt;
                &lt;title&gt;Prime Minister to Fly Out Tomorrow&lt;/title&gt;
                &lt;meta property="dc:creator"&gt;John Doe&lt;/meta&gt;
                &lt;/meta&gt;
        &lt;/head&gt;
        &lt;body&gt;
                &lt;h1&gt;Prime Minister to Fly Out Tomorrow&lt;/h1&gt;
                &lt;span&gt;By John Doe&lt;/span&gt;
                &lt;p&gt;
                        Tomorrow the &lt;span resource="p:TonyBlair"&gt;Prime Minister&lt;/span&gt; is expected to fly to ...
                &lt;/p&gt;
        &lt;/body&gt;
&lt;/html&gt;
                        </pre>
</div>

<p>This often seems like unnecessary duplication, and may add an additional
maintenance headache. However, specifying the <code>property</code>
explicitly rather than using the default, we can abbreviate this document
to:</p>

<div class="exampleInner">
<pre>&lt;html xmlns:dc="http://purl.org/dc/elements/1.1/"&gt;
        &lt;head&gt;
                &lt;title&gt;Prime Minister to Fly Out Tomorrow&lt;/title&gt;
        &lt;/head&gt;
        &lt;body&gt;
                &lt;h1&gt;Prime Minister to Fly Out Tomorrow&lt;/h1&gt;
                &lt;span&gt;By
                        &lt;span property="dc:creator"&gt;John Doe&lt;/span&gt;
                &lt;/span&gt;
                &lt;p&gt;
                        Tomorrow the &lt;span resource="p:TonyBlair"&gt;Prime Minister&lt;/span&gt; is expected to fly to ...
                &lt;/p&gt;
        &lt;/body&gt;
&lt;/html&gt;
                                </pre>
</div>

<table border="1" summary="Editorial note">
  <tbody>
    <tr>
      <td align="left" valign="top" width="50%"><b>Editorial note</b></td>
      <td align="right" valign="top" width="50%">&nbsp;</td>
    </tr>
    <tr>
      <td colspan="2" align="left" valign="top">Note that this means we have
        to refine the rule earlier, in which there was an implied
        "xhtml2:reference" value. We should now say that this is the default
        value for <code>properrty</code>.</td>
    </tr>
  </tbody>
</table>
</div>

<div class="div2">
<h3><a name="div254348288" id="div254348288"></a>4.4 Representing Value</h3>

<p>Just as we sometimes need to refer to a resource, so sometimes we need to
indicate clearly what the vallue of something is. In this example:</p>

<div class="exampleInner">
<pre>Yesterday in Parliament the &lt;span resource="p:WinstonChurchill"&gt;Prime Minister&lt;/span&gt;
said that we will fight on the beaches ...
                                </pre>
</div>

<p>we would like to indicate that the speech was made on June 4th, 1940,
since this would allow sophisticated metadata searches to be made. To do this
we use the attributes <code>val</code> and <code>datatype</code>:</p>

<div class="exampleInner">
<pre>&lt;span datatype="xsd:date" val="1940-06-04"&gt;Yesterday&lt;/span&gt; in Parliament the &lt;span resource="p:WinstonChurchill"&gt;Prime Minister&lt;/span&gt;
said that we will fight on the beaches ...
                                </pre>
</div>

<p>This would create the following triples:</p>

<div class="exampleInner">
<pre>&lt;&gt; xhtml2:reference "1940-06-04"^^xsd:date , p:WinstonChurchill .
                                </pre>
</div>
</div>
</div>

<div class="div1">
<h2><a name="div154351016" id="div154351016"></a>5 Bibliography</h2>
<dl>
  <dt class="label"><a name="XHTML-2.0-LINKTYPES"
  id="XHTML-2.0-LINKTYPES"></a>XHTML-2.0-LINKTYPES</dt>
    <dd>XHTML 2.0 Link Types  (See <a
      href="http://www.w3.org/TR/2003/WD-xhtml2-20030506/abstraction.html#dt_LinkTypes">http://www.w3.org/TR/2003/WD-xhtml2-20030506/abstraction.html#dt_LinkTypes</a>.)</dd>
  <dt class="label"><a name="N3-PRIMER" id="N3-PRIMER"></a>N3-PRIMER</dt>
    <dd>N3 Primer  (See <a
      href="http://www.w3.org/2000/10/swap/Primer">http://www.w3.org/2000/10/swap/Primer</a>.)</dd>
</dl>
</div>
</div>
</body>
</html>