 | Level: Introductory Micah Dubinko (micah@brainattic.info), Principal Consultant, Brain Attic, L.L.C.
04 Mar 2005 As a fundamental part of the Web, hypertext linking has been the subject of repeated attempts at standardization beyond the basic format allowed in simple HTML. Such attempts can be characterized as efforts to balance machine processing ability with authoring convenience. The latest specification in this area, XHTML 2.0, just might have gotten it right.
The one thing basically all Web sites have in common is hyperlinks.
Beginning and advanced Web surfers alike rely on links to navigate
around the Web. From the earliest days of XML, the standards-builders
always considered linking an essential part of their overall story
arc -- in fact, the linking specification was once called "Extensible
Markup Language (XML): Part 2. Linking" (see Resources).
Web authors are familiar with simple linking markup like that in
Listing 1, where href attributes create links
that the user can choose to follow, and src
attributes create links that usually load automatically.
Listing 1. HTML linking
<a href="http://example.com" rel="example"
title="link to a remote page">
<img src="http://example.info/img.png"
alt="link to a remote image" />
</a>
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XHTML version 2.0 preserves the basic approach here, but adds a new
twist. The following information is based on the XHTML 2.0 Working Draft
of 22 July 2004.
Linking: From any element
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Why not XLink?
The W3C has a Recommendation called XML Linking Language (XLink -- see Resources) intended to
be used for hypertext linking. However, XHTML in any version has been
deemed incompatible with XLink. Still, XHTML has adopted the useful
terminology and semantics defined in that specification.
XLink helps specify when a link activation occurs, including
onLoad for typical images on a Web page,
and onRequest for manually-activated
hyperlinks. XLink also helps specify link behavior, including replace for hyperlinks that replace the current
page, new for pop-up windows, and embed for things like images. XLink contains
additional wording to ensure that these concepts can apply to
contexts beyond basic visual browsers, including voice and accessibility
applications.
So, what is incompatible about XLink? Much has been written about
this (for a sampling, see Resources), but in short XLink expects all
links to be named xlink:href, with the prefix
mapped to a namespace in the usual fashion. As you will see in this
article, XHTML 2.0 requires different kinds of links with different
names, often on the same element.
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Listing 1 uses the a element -- which stands
for an "anchor" -- to define an outgoing link. Most people, however, think
about anchors as link targets, not sources. Only by historical accident
has the a element come to be the de facto way
to specify links. In other words, there?s no good reason why other
elements shouldn't also be able to serve as link ends. XHTML 2.0
dispenses with this limitation and broadly allows href along with several other
attributes.
A table cell, an image, a list item -- anything can be a link. Just add
an href attribute and put the destination URL
as the value. In fact, one of the several attribute collections defined
by XHTML 2.0 is the Hypertext attribute collection, which
applies to every element in XHTML 2.0.
The Hypertext attribute
collection
Besides href, XHTML 2.0 supplies a
collection of additional attributes to further augment the description
of a link. These attributes, which are likely to change slightly by the
time XHTML 2.0 reaches Last Call, are available in all the same places
that href can be used:
hreflang indicates the expected
language at the remote end of the link. It may be a space-separated
list.hreftype indicates the media type of
the link destination; whether it is, say, SVG or XHTML or some other
type. It too may be a space-separated list.cite designates a source document
reference for some text, typically some kind of a quotation.
Previously, this attribute existed only on elements like blockquote. Now that it is widely available,
authors are thinking about richer ways to interconnect their
hypertext. The interesting part is that the XHTML specification
says that user agents
"should" make this link available somehow. The details are left to each
piece of software, but I'd expect to see this kind of link show up on
a right-click menu in more-innovative browsers.access (which is quite different from accesskey in previous versions) assigns short names, like "contents", to various parts of a document.
Accessibility tools can then handle those parts appropriately.- The
nextfocus and prevfocus attributes contain IDREFs of other
elements. Taken as a whole, these form chains that guide navigation
through the document, and they work for more than just anchors and
form controls. The default navigation order is the same as the order
in which the elements appear in the document; these attributes allow
fine-tuning of navigation order. Additionally, if the XHTML document reference includes a fragment -- say http://search.example.com#query -- and the document includes a focusable element with an id
of query, that element receives initial focus
and serves as the starting point for further navigation.
target and xml:base are largely unchanged in XHTML 2.0. Web
authors might be accustomed to specifying target="_new" to specify that a link opens in a
new window. XHTML 2.0 no longer hard-codes any special values, and
instead defers to a related specification called XFrames (see
Resources) to define how linked-to resources fit into the overall
environment. The xml:base attribute sets up
a context in which relative URLs are interpreted.
One good use for the Hypertext attribute collection is with
navigation lists, a new feature in XHTML 2.0. Listing 2 illustrates:
Listing 2. Navigation list
<nl xml:base="http://xformsinstitute.com/essentials/browse/">
<label>Table of Contents</label>
<li href="ch01.php">Introduction to Web Forms</li>
<li href="ch02.php">XForms Building Blocks</li>
<li href="ch03.php">XPath in XForms</li>
...
</nl>
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HLink: An alternate solution?
In response to apparent shortcomings of XLink, the HTML Working Group
drafted a proposal called HLink (see Resources), which leveraged the
good work in XLink in a way that could be mapped to arbitrary attributes
on any element. HLink had some interesting additions, like a new actuate value of onRequestSecondary for right-mouse clicks or the
equivalent, and ways to handle things like form submission and error
handling that weren't clear with XLink by itself.
In the end, though, HLink wasn't an appropriate solution. The
terminology and semantics from XLink could be used directly, without the
need for a separate layer in between.
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Navigation lists help authors direct users through their Web sites, something that many developers have tried to accomplish through forms.
However, the explicit hyperlink markup is a more natural -- and more accessible -- fit for conveying navigation
information.
The Embedding attribute collection
So far, all the links that I've discussed are href-style links, which the user clicks to make things
happen. But you'll also encounter links that load immediately -- especially
images. For these, XHTML 2.0 defines the Embedding
attribute collection.
The showcase attribute in this collection is src, which bears the URL of the remote resource,
and normally gets traversed while the main document is loading; it requires no
special action from the user. The other attribute, type, provides additional metadata about
what?s expected at the other end.
One key design feature that?s easy to miss: If
it can be loaded, the linked-to content replaces the element in
question; in other words, a fallback mechanism is built-in, even over
multiple levels of nesting. Listing 3 shows how this works.
Listing 3. Embedding fallback
<p href="http://scp.example.info" src="cow.jpg" usemap="#map1">
<nl>
<li href="scp.html"/>
<li href="/"/>
</nl>
</p>
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In Listing 3, a text-only browser, or a graphical browser with images
turned off sees a navigation list, but everyone else sees the
interactive image map defined on the p
element, the details of which are specified in map1. This fallback behavior is used to good effect
with the object element, as shown in Listing
4.
Listing 4. Object fallback
<!-- First, try Flash -->
<object src="earthtime.swf" type="application/x-shockwave-flash">
<!-- Else, try the image -->
<object src="earthtime.jpg" type="image/jpeg"/">
<!-- Else, alternate text -->
A map of the earth showing night and day
</object>
</object>
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Browsers encountering the code in Listing 4 first try to load
the Flash applet. If that fails for any reason, the browser attempts
to load the JPG image. If that too fails, then the browser displays the innermost alternate text. Web users welcome this kind of flexibility.
Are we there yet?
It?s safe to say that standards development around hypertext linking
hasn't worked out the way anyone initially envisioned it. But at last,
it looks like progress is being made. XHTML 2.0 goes to great lengths
to remain friendly to authors who learn mainly through their browser?s
View source feature, while adding power and flexibility.
XHTML 2.0 is approaching a Last Call period, during which time the
Working Group will solicit constructive comments. If other W3C efforts
serve as a reliable guide, this could be a lengthy and possibly
contentious period. But even today, XHTML 2.0 has a positive
effect as it stimulates linking-related discussions within the Web
development community. All the right people are now aware of the issues,
and talking things through, and that?s good news.
Only time will tell what the end result will be. Yet many are
hopeful that XHTML 2.0 will be a step towards a better Web.
Resources - Check out Robin Cover?s archive, which includes a thorough compendium of documents
on XML and linking. So closely were these topics tied together that
many of the earlier links appear under the XML page. Others appear
under the XLL
page.
- Get an alternate viewpoint on XHTML linking through "A
Hyperlink Offering," the author?s fictional account of a discussion between
two literary characters about some of the challenges of hypertext linking.
- Go straight to the source and read the XLink 1.0 W3C
Recommendation.
- Next, compare notes with the HLink Working Draft, dated
September 2002 (and not likely to be updated again).
- Finally, read the latest XHTML 2.0 Working
Draft to see the current status of linking in XHTML.
- Read about the replacement for frames (that?s often used with the
target attribute), XFrames, a W3C Working Draft
that works alongside XHTML 2.0.
- See if new specifications or documents are under development at
the W3C -- consult the Technical Reports
page.
- Read the full text of the O?Reilly book XForms
Essentials online. XForms takes a similar approach to linking
as XHTML 2.0. You can also order the book from the
developerWorks Developer Bookstore.
- Want a more complete understanding of how all the major XML standards interrelate?
Check out Uche Ogbuji?s excellent four-part survey of XML standards here on developerWorks:
- Find more XML resources on the developerWorks XML zone.
- Learn how you can become an IBM Certified Developer in XML and related technologies.
About the author | | | Micah Dubinko is a consultant and founder of Brain Attic, L.L.C., a software vendor and consultancy specializing in defeating information overload. He wrote XForms Essentials for O'Reilly Media and served on the Working Group that developed XForms 1.0. He lives and works in Phoenix, AZ. You can contact him at micah@brainattic.info. |
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