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
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>
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
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
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
A table cell, an image, a list item -- anything can be a link. Just add
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
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
href can be used:
hreflangindicates the expected language at the remote end of the link. It may be a space-separated list.
hreftypeindicates 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.
citedesignates 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
accesskeyin previous versions) assigns short names, like "contents", to various parts of a document. Accessibility tools can then handle those parts appropriately.
prevfocusattributes 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
query, that element receives initial focus and serves as the starting point for further navigation.
xml:baseare 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:baseattribute 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>
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
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>
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
element, the details of which are specified in
map1. This fallback behavior is used to good effect
object element, as shown in Listing
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>
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.
- 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
targetattribute), 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:
- Part 1 -- The core standards (January 2004)
- Part 2 -- XML processing standards (February 2004)
- Part 3 -- The most important vocabularies (February 2004)
- Part 4 -- Detailed cross-reference of the most important XML standards (March 2004)
- Find more XML resources on the developerWorks XML zone.
- Learn how you can become an IBM Certified Developer in XML and related technologies.