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6.7 Relationships
Very few documents stand alone.
Instead, a document is usually part of a collection of documents,
each connected by one or several of the hypertext strands we describe
in this chapter. One document may be a part of several collections,
linking to some documents and being linked to by others. Readers move
between the document families as they follow the links that interest
them.
When you link two documents, you establish an explicit relationship
between them. Conscientious authors use the rel
attribute of the <a> tag to indicate the
nature of the link. In addition, two other tags may be used within a
document to further clarify the location of a document within a
document family and its relationship to the other documents in that
family. These tags, <base> and
<link>, are placed within the body of the
<head> tag. [<head>]
6.7.1 The <base> Header Element
As we previously explained, URLs within a
document can be either absolute (with every element of the URL
explicitly provided by the author) or relative (with certain elements
of the URL omitted and supplied by the browser). Normally, the
browser fills in the blanks of a relative URL by drawing the missing
pieces from the URL of the current document. You can change that with
the <base> tag.
<base>Function Defines the base URL for other anchors in the document Attributes href, target End tag None in HTML; </base> or <base ... /> in XHTML Contains Nothing Used in head_content |
document header, not in its body contents. The browser thereafter
uses the specified base URL, not the current
document's URL, to resolve all relative URLs,
including those found in <a>,
<img>, <link>, and
<form> tags. It also defines the URL that
will be used to resolve queries in searchable documents containing
the <isindex> tag. [Section 6.2]
6.7.1.1 The href attribute
The href
attribute must have a valid URL as its value, which the browser then
uses to define the absolute URL against which relative URLs are based
within the document.
For example, the <base> tag in this XHTML
document head:
<head>
<base href="http://www.kumquat.com/" />
</head>
...
tells the browser that any relative URLs within this document are
relative to the top-level document directory on
www.kumquat.com, regardless of the address and
directory of the machine from which the user retrieved the current
document.
Contrary to what you may expect, you can make the base URL relative,
not absolute. The browser should (but doesn't
always) form an absolute base URL out of this relative URL by filling
in the missing pieces with the URL of the document itself. This
property can be used to good advantage. For instance, in this next
HTML example:
<head>
<base href=">
</head>
...
the browser makes the <base> URL into one
relative to the server's /info
directory, which probably is not the same directory of the current
document. Imagine if you had to re-address every link in your
document with that common directory. Not only does the
<base> tag help you shorten those URLs in
your document that have a common root, it also lets you constrain the
directory from which relative references are retrieved without
binding the document to a specific server.
6.7.1.2 The target attribute
When
working with documents inside frames, the target attribute with the
<a> tag ensures that a referenced URL gets
loaded into the correct frame. Similarly, the
target attribute for the
<base> tag lets you establish the default
name of one of the frames or windows in which the browser is to
display redirected hyperlinked documents. [Section 11.1]
If you have no other default target for your hyperlinks within your
frames, you may want to consider using <base
target=_top>. This ensures that links that are
not specifically targeted to a frame or window will load in the
top-level browser window. This eliminates the embarrassing and common
error of having references to pages on other sites appear within a
frame on your pages, instead of within their own pages. A minor bit
of HTML, to be sure, but it makes life much easier for your readers.
6.7.1.3 Using <base>
The most important reason for using <base>
is to ensure that any relative URLs within the document will resolve
into correct document addresses, even if the documents themselves are
moved or renamed. This is particularly important when creating a
document collection. By placing the correct
<base> tag in each document, you can move
the entire collection between directories and even servers without
breaking all of the links within the documents. You also need to use
the <base> tag for a searchable document
(<isindex>) if you want user queries posed
to a URL different from that of the host document.
A document that contains both the <isindex>
tag and other relative URLs may have problems if the relative URLs
are not relative to the desired index-processing URL. Since this is
usually the case, don't use relative URLs in
searchable documents that use the <base> tag
to specify the query URL for the document.
6.7.2 The <link> Header Element
Use the
<link> tag to define the relationship
between the current document and another in a web collection.
<link>Function Defines a relationship between this document and another document Attributes charset, class, dir, href, hreflang, id, lang, media, onClick, onDblClick, onKeyDown, onKeyPress, onKeyUp, onMouseDown, onMouseMove, onMouseOut, onMouseOver, onMouseUp, rel, rev, style, target, title, type End tag None in HTML; </link> or <link ... /> in XHTML Contains Nothing Used in head_content |
<head> content and nowhere else. The
attributes of the <link> tag are used like
those of the <a> tag, but their effects
serve only to document the relationship between documents. The
<link> tag has no content, and only XHTML
supports the closing </link> tag.
6.7.2.1 The href attribute
As
with its other tag applications, the href
attribute specifies the URL of the target
<link> tag. It is a required attribute, and
its value is any valid document URL. The specified document is
assumed to have a relationship to the current document.
6.7.2.2 The rel and rev attributes
The rel and
rev
attributes express the relationship between the source and target
documents. The rel attribute specifies the
relationship from the source document to the target; the
rev attribute specifies the relationship from the
target document to the source document. Both attributes can be
included in a single <link> tag.
The value of either attribute is a space-separated list of
relationships. The actual relationship names are not specified by the
HTML standard, although some have come into common usage, as listed
in Section 6.3.1.5. For example, a document that
is part of a sequence of documents might use:
<link href=" rel=next rev=prev>
when referencing the next document in the series. The relationship
from the source to the target is that of moving to the next document;
the reverse relationship is that of moving to the previous document.
6.7.2.3 The title attribute
The
title
attribute lets you specify the title of the document to which you are
linking. This attribute is useful when referencing a resource that
does not have a title, such as an image or a non-HTML document. In
this case, the browser might use the <link>
title when displaying the referenced document. For example:
<link href="/image/library/english/10232_kumquat.gif"
title="A photograph of the Noble Fruit">
tells the browser to use the indicated title when displaying the
referenced image.
The value of the attribute is an arbitrary character string, enclosed
in quotation marks.
6.7.2.4 The type attribute
The type
attribute provides the MIME content type of the linked document.
Supported by both Internet Explorer and Netscape, the HTML 4 and
XHTML standard type attribute can be used with any
linked document. It is often used to define the type of a linked
style sheet. In this context, the value of the
type attribute is usually
text/css. For example:
<link href="/image/library/english/10232_classic.css" rel=stylesheet type="text/css">
creates a link to an external style sheet within the
<head> of a document. See Chapter 8 for details.
6.7.2.5 How browsers might use <link>
Although the
standards do not require browsers to do anything with the information
provided by the <link> tag,
it's not hard to envision how this information might
be used to enhance the presentation of a document.
As a simple example, suppose you consistently provide
<link> tags for each of your documents that
define next, prev, and
parent links. A browser could use this information
to place at the top or bottom of each document a standard toolbar
containing buttons that would jump to the appropriate related
document. By relegating the task of providing simple navigational
links to the browser, you are free to concentrate on the more
important content of your document.
As a more complex example, suppose that a browser expects to find a
<link> tag defining a glossary for the
current document and that this glossary document is itself a
searchable document. Whenever a reader clicked on a word or phrase in
the document, the browser could automatically search the glossary for
the definition of the selected phrase, presenting the result in a
small pop-up window.
As the Web evolves, expect to see more and more uses of the
<link> tag to define document relationships
explicitly.
6.7.2.6 Other <link> attributes
The HTML 4 and XHTML standards also include the ubiquitous collection
of attributes related to style sheets and user events, and language
for the <link> tag. You can refer to the
corresponding section describing these attributes for the
<a> tag for a complete description of their
usage. [Section 6.3.1]
Since you put the <link> tag in the
<head> section, whose contents are not
displayed, it may seem that these attributes are useless. It is
entirely possible that some future browser may find some way to
display the <link> information to the user,
possibly as a navigation bar or a set of hot-list selections. In
those cases, the display and rendering information would prove
useful. Currently, no browser provides these capabilities.
