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2.7 Hyperlinks
While
text may be the meat and bones of an HTML or XHTML document, the
heart is hypertext. Hypertext gives users the ability to retrieve and
display a different document in their own or someone
else's collection simply by a click of the keyboard
or mouse on an associated word or phrase (hyperlink
) in the document. Use these interactive hyperlinks to
help readers easily navigate and find information in your own or
others' collections of otherwise separate documents
in a variety of formats, including multimedia, HTML, XHTML, other
XML, and plain ASCII text. Hyperlinks literally bring the wealth of
knowledge on the whole Internet to the tip of the mouse pointer.
To include a hyperlink to some other document in your own collection
or on a server in Timbuktu, all you need to know is the
document's unique address and how to drop an
anchor into your document.
2.7.1 URLs
While it is hard to believe, given the
millions, perhaps billions, of them out there, every document and
resource on the Internet has a unique address, known as its
uniform resource locator (URL; commonly
pronounced "you-are-ell"). A URL
consists of the document's name preceded by the
hierarchy of directory names in which the file is stored
(pathname),
the Internet domain name of the server that
hosts the file, and the software and manner by which the browser and
the document's host server communicate to exchange
the document (protocol
):
protocol://server_domain_name/pathname
Here are some sample URLs:
http://www.kumquat.com/docs/catalog/price_listl
price_listl
http://www.kumquat.com/
ftp://ftp.netcom.com/pub/
The first example is an
absolute or complete URL. It includes every part of
the URL format: protocol, server, and the pathname of the document.
While absolute URLs leave nothing to the imagination, they can lead
to big headaches when you move documents to another directory or
server. Fortunately, browsers also let you use
relative URLs and automatically fill in any
missing portions with respective parts from the current
document's
base URL. The second example is the simplest
relative URL of all; with it, the browser assumes that the
price_listl document is located on the same
server, in the same directory as the current document, and uses the
same network protocol.
Relative URLs are also useful if you don't know a
directory or document's name. The third URL example,
for instance, points to
kumquat.com's web home page. It
leaves it up to the kumquat server to decide what file to send along.
Typically, the server delivers the first file in the directory, one
named indexl, or simply a listing of the
directory's contents.Although appearances may deceive, the last FTP example URL actually
is absolute; it points directly at the contents of the
/pub directory.
2.7.2 Anchors
The anchor
(<a>) tag is the HTML/XHTML feature for
defining both the source and the destination of a
hyperlink.[7] You'll most often see and use the
<a> tag with its
href attribute to define a source
hyperlink. The value of the href attribute is the
URL of the destination.
[7] The nomenclature here is a bit
unfortunate: the "anchor" tag
should mark just a destination, not the jumping-off point of a
hyperlink, too. You "drop anchor";
you don't jump off one. We won't
even mention the atrociously confusing terminology the W3C uses for
the various parts of a hyperlink, except to say that someone got
things all "bass ackwards."
The contents of the source <a> tag the
words and/or images between it and its </a>
end tag is the portion of the document that is specially
activated in the browser display and that users select to take a
hyperlink. These anchor contents usually look
different from the surrounding content (text in a different color or
underlined, images with specially colored borders, or other effects),
and the mouse-pointer icon changes when passed over them. The
<a> tag contents, therefore, should be text
or an image (icons are great) that explicitly or intuitively tells
users where the hyperlink will take them. [Section 6.3.1]
For instance, the browser will specially display and change the mouse
pointer when it passes over the "Kumquat
Archive" text in the following example:
For more information on kumquats, visit our
<a href="http://www.kumquat.com/archivel">
Kumquat Archive</a>
If the user clicks the mouse button on that text, the browser
automatically retrieves from the server
www.kumquat.com a web
(http:) page named
archivel, then displays it for the user.
2.7.3 Hyperlink Names and Navigation
Pointing to another document
in some collection somewhere on the other side of the world is not
only cool, it also supports your own web documents. Yet the
hyperlink's chief duty is to help users navigate
your collection in their search for valuable information. Hence, the
concept of the home page and supporting documents has arisen.
None of your documents should run on and on. First,
there's a serious performance issue: the value of
your work suffers, no matter how rich it is, if the document takes
forever to download and if, once it is retrieved, users must
endlessly scroll up and down through the display to find a particular
section.
Rather, design your work as a collection of several compact and
succinct pages, like chapters in a book, each focused on a particular
topic for quick selection and browsing by the user. Then use
hyperlinks to organize that collection.
For instance, use your home page the leading document of
the collection as a master index full of brief descriptions and
respective hyperlinks to the rest of your collection.
You can also use either the name variant of the
<a> tag or the id
attribute of nearly all tags to specially identify sections of your
document. Tag ids and name
anchors serve as internal hyperlink targets in your documents to help
users easily navigate within the same document or jump to a
particular section within another document. Refer to that
id'd section in a hyperlink by
appending a pound sign (#) and the section name as
the suffix to the URL.
For instance, to reference a specific topic in an archive, such as
"Kumquat Stew Recipes" in our
example Kumquat Archive, first mark the section title with an
id:
...preceding content...
<h3 id="Stews">Kumquat Stew Recipes</h3>
in the same or another document, then prepare a source hyperlink that
points directly to those recipes by including the
section's id value as a suffix to
the document's URL, separated by a pound sign:
For more information on kumquats, visit our
<a href="http://www.kumquat.com/archivel">
Kumquat Archive</a>,
and perhaps try one or two of our
<a href="http://www.kumquat.com/archivel#Stews">
Kumquat Stew Recipes</a>.
If selected by the user, the latter hyperlink causes the browser to
download the archivel document and start the
display at our "Stews" section.
2.7.4 Anchors Beyond
Hyperlinks are not limited to other HTML and XHTML documents. Anchors
let you point to nearly any type of document available over the
Internet, including other Internet services.
However, "let" and
"enable" are two different things.
Browsers can manage the various Internet services, like FTP and
Gopher, so that users can download non-HTML documents. They
don't yet fully or gracefully handle
multimedia.
Today, there are few standards for the many types and formats of
multimedia. Computer systems connected to the Web vary wildly in
their abilities to display those sound and video formats. Except for
some graphics images, standard HTML/XHTML gives you no specific
provision for display of multimedia documents except the ability to
reference one in an anchor. The browser, which retrieves the
multimedia document, must activate a special
helper application, download and execute
an associated applet, or have a
plug-in accessory installed to decode and
display it for the user right within the document's
display.
Although HTML and most web browsers currently avoid the confusion by
sidestepping it, that doesn't mean you
can't or shouldn't exploit
multimedia in your documents: just be aware of the
limitations.
