LINK - Document Relationship

Syntax

<LINK>

Attribute Specifications

REL=LinkTypes (relationship to link)
REV=LinkTypes (relationship from link)
HREF=url (hypertext reference)
TYPE=ContentType (content-type of link)
TARGET=FrameTarget (frame to render link in)
MEDIA=MediaDesc (media appropriate for link)
HREFLANG=LanguageCode (language of link)
CHARSET=Charset (character encoding of link)
[4.0] ID=string
[4.0] CLASS=string
[4.0] STYLE=string
[4.0] TITLE=string
[4.0] LANG=Language (i.e. RU - Russian)
[4.0] DIR=ltr|rtl
[4.0] SCRIPTING EVENTS=string

Contents
Empty
Contained in

HEAD

Description

The LINK element defines document relationships. Any number of LINK elements may be contained in the HEAD of a document. Many browsers lack support for LINK, so authors should not depend on the browser making the links available to the user.

REL, REV, TYPE & HREF

The REL and REV attributes define the nature of the relationship between the documents and the linked resource. REL defines a link relationship from the current document to the linked resource while REV defines a relationship in the opposite direction. For example,

<LINK REL=Glossary HREF="foo.html">

indicates that foo.html is a glossary for the current document while

<LINK REV=Subsection HREF="bar.html">

indicates that the current document is a subsection of bar.html. The value of the REL and REV attributes is a space-separated list of link types.

Commonly used relationships include the next or previous document in a sequence, the starting page in a collection of documents, a document with copyright information, and information about the author. A document could define these relationships as follows:

<LINK REL=Prev HREF="base.html" TITLE="BASE - Document Base url">
<LINK REL=Next HREF="meta.html" TITLE="META - Metadata">
<LINK REL=Start HREF="" TITLE="HTML 4.0 Reference">
<LINK REL=Copyright HREF="/copyright.html" TITLE="Copyright Notice">
<LINK REV=Made HREF="mailto:liam@htmlhelp.com" TITLE="HTML 4.0 Reference Feedback">

While the value of REL and REV is case-insensitive, the Lynx browser renders the relationship exactly as given by the author. Authors should therefore be consistent in their case, and may wish to capitalize the first letter while using lowercase for the rest.

Authors can also use the LINK element to apply an external style sheet. REL=StyleSheet specifies a persistent or preferred style while REL="Alternate StyleSheet" defines an alternate style. A persistent style is one that is always applied when style sheets are enabled. The absence of the TITLE attribute indicates a persistent style.

A preferred style is one that is automatically applied. The combination of REL=StyleSheet and a TITLE attribute specifies a preferred style. Authors cannot specify more than one preferred style.

An alternate style is indicated by REL="Alternate StyleSheet". The user could choose to replace the preferred style sheet with an alternate one, though current browsers generally lack the ability to choose alternate styles.

A single style may also be given through multiple style sheets:

<LINK REL=StyleSheet HREF="basics.css" TITLE="Contemporary" TYPE="text/css">
<LINK REL=StyleSheet HREF="tables.css" TITLE="Contemporary" TYPE="text/css">
<LINK REL=StyleSheet HREF="forms.css" TITLE="Contemporary" TYPE="text/css">

In this example, three style sheets are combined into one "Contemporary" style that is applied as a preferred style sheet. To combine multiple style sheets into a single style, each style sheet's LINK must use the same TITLE.

MEDIA

LINK's MEDIA attribute specifies the media for which the linked resource is designed. With REL=StyleSheet, this allows authors to restrict a style sheet to certain output devices, such as printers or aural browsers. The attribute's value is a comma-separated list of media descriptors. The following media descriptors are defined in HTML 4.0 and are case-sensitive:

Netscape Navigator 4.x incorrectly ignores any style sheet linked with a MEDIA value other than screen. For example, MEDIA="screen, projection" will cause the style sheet to be ignored by Navigator 4.x, even if the presentation device is a computer screen. Navigator 4.x also ignores style sheets declared with MEDIA=all. Most other browsers ignore the MEDIA attribute.

HREFLANG & CHARSET

The optional HREFLANG and CHARSET attributes of LINK give the language and character encoding, respectively, of the link. The language should be specified according to RFC 1766; examples include en for English, en-US for American English, and ja for Japanese. Examples of character encodings include ISO-8859-1, SHIFT_JIS, and UTF-8.

The Alternate link relationship defines an alternate version of the document. Translations of a page can be identified by using REL=Alternate along with the HREFLANG attribute. Versions of the page tailored for specific media can be provided by combining REL=Alternate with the MEDIA attribute. Some examples follow:

<LINK REL=Alternate HREF="index.fr.html" HREFLANG=fr LANG=fr TITLE="Version française">
<LINK REL=Alternate HREF="index.ja.html" HREFLANG=ja CHARSET="SHIFT_JIS" TITLE="Japanese version">
<LINK REL=Alternate HREF="/distribution/html40.pdf" TYPE="application/pdf" MEDIA=print TITLE="PDF version">

Note that the LANG and DIR attributes apply to the text of the TITLE attribute, not to the content of the link.

TARGET

The TARGET attribute is used with frames to specify in which frame the link should be rendered. If no frame with such a name exists, the link is rendered in a new window unless overridden by the user. Special frame names begin with an underscore:

[4.0] ID

The ID attribute uniquely identifies an element within a document. No two elements can have the same ID value in a single document. The attribute's value must begin with a letter in the range A-Z or a-z and may be followed by letters (A-Za-z), digits (0-9), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").

The following example uses the ID attribute to identify each of the first two paragraphs of a document:

<P ID=firstp>My first paragraph.</P>
<P ID=second>My second paragaph.</P>

The paragraphs in the example could have style rules associated with them through their ID attributes. The following Cascading Style Sheet defines unique colors for the two paragraphs:

P#firstp {
  color: navy;
  background: transparent
}

P#secondp {
  color: black;
  background: transparent
}

The paragraphs in the initial example could also be used as a target anchor for links:

<P>See <A HREF="#firstp">the opening paragraph</A> for more information.</P>

Note that most browsers do not support the ID attribute for link anchors. For current browsers, authors should use <A NAME>...</A> within the element instead of ID.

Since ID and NAME share the same name space, authors cannot use the same value for an ID attribute and a NAME attribute in the same document. Also note that while NAME may contain entities, the ID attribute value may not.

[4.0] CLASS

The CLASS attribute specifies the element to be a member of one or more classes. Classes allow authors to define specific kinds of a given element. For example, an author could use <CODE CLASS=Java> when giving Java code and <CODE CLASS=Perl> when giving Perl code.

Unlike with the ID attribute, any number of elements can share the same class. An element may also belong to multiple classes; the CLASS attribute value is a space-separated list of class names.

Note that most current browsers do not support multiple classes. Such browsers typically ignore a CLASS attribute that specifies multiple classes.

The CLASS attribute is particularly useful when combined with style sheets. For example, consider the following navigation bar:

<DIV CLASS=navbar>
<P><A HREF="/">Home</A> | <A HREF="./">Index</A> | <A HREF="/search.html">Search</A></P>
<P><A HREF="/"><IMG SRC="logo.gif" ALT="" TITLE="WDG Logo"></A></P>
</DIV>

This example's use of the CLASS attribute allows style rules to easily be added. The following Cascading Style Sheet suggests a presentation for the preceding example:

.navbar {
  margin-top: 2em;
  padding-top: 1em;
  border-top: solid thin navy
}

.navbar IMG { float: right }

@media print {
  .navbar { display: none }
}
[4.0] STYLE

The STYLE attribute allows authors to specify style rules inline for a single occurrence of an element. An example follows:

<P>A popular font for on-screen reading is <SPAN STYLE="font-family: Verdana">Verdana</SPAN>.</P>

When the STYLE attribute is used, a default style sheet language must be specified for the document by setting the Content-Style-Type HTTP header to the media type of the style sheet language. The previous example could use the following META element in the document's HEAD:

<META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css">

In most cases, use of the CLASS or ID attributes is a better choice than using STYLE since ID and CLASS can be selectively applied to different media and since they provide a separation of content and presentation that often simplifies maintenance.

[4.0] TITLE

The TITLE attribute provides a title for an element and is commonly implemented as a "tooltip" on visual browsers, though many browsers lack support for TITLE. The attribute is most useful with A, LINK, IMG, and OBJECT elements, where it provides a title for the linked or embedded resource. Some examples follow:

TITLE is also helpful with the ABBR and ACRONYM elements to provide the long form of the abbreviation. Examples:

Internationalization Attributes

[4.0] LANG

The LANG attribute specifies the language of an element's attribute values and its content, including all contained elements that do not specify their own LANG attribute. While the LANG attribute is not widely supported, its use may help search engines index a document by its language while allowing speech synthesizers to use language-dependent pronunciation rules. As well, visual browsers can use the language's proper quotation marks when rendering the Q element.

The attribute value is case-insensitive, and should be specified according to RFC 1766; examples include en for English, en-US for American English, and ja for Japanese. Whitespace is not allowed in the language code.

Use of the LANG attribute also allows authors to easily change the style of text depending on the language. For example, a bilingual document may have one language in italics if rendered visually or a different voice if rendered aurally. The HTML of such a document might be as follows:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"
   "http://www.w3.org/TR/REC-html40/strict.dtd">
<TITLE>Welcome - Bienvenue</TITLE>
<H1>
  <SPAN LANG=en>Welcome</SPAN> -
  <SPAN LANG=fr>Bienvenue</SPAN>
</H1>
<P LANG=en>This paragraph is in English.</P>
<P LANG=fr>Ce paragraphe est en français.</P>
...

A document's primary language may be set using the LANG attribute on the HTML element, or, alternatively, by using the Content-Language HTTP header.

[4.0] DIR

The DIR attribute specifies the directionality of text--left-to-right (DIR=ltr, the default) or right-to-left (DIR=rtl). Characters in Unicode are assigned a directionality, left-to-right or right-to-left, to allow the text to be rendered properly. For example, while English characters are presented left-to-right, Hebrew characters are presented right-to-left.

Unicode defines a bidirectional algorithm that must be applied whenever a document contains right-to-left characters. While this algorithm usually gives the proper presentation, some situations leave directionally neutral text and require the DIR attribute to specify the base directionality.

Text is often directionally neutral when there are multiple embeddings of content with a different directionality. For example, an English sentence that contains a Hebrew phrase that contains an English quotation would require the DIR attribute to define the directionality of the Hebrew phrase. The Hebrew phrase, including the English quotation, should be contained within a SPAN element with DIR=rtl.

[4.0] Scripting Events

A number of attributes that define client-side scripting events are common to most elements. The attribute value is a script--typically a function call or a few short statements--that is executed when the event occurs. The value may contain entities (e.g., &quot;).

The following example features JavaScript code to handle two events of a submit button, giving the user a reminder in the status bar when the mouse moves over the button and clearing the status bar when the mouse moves away. Note that the attribute values are delimited by single quotes since double quotes are used within them.

<INPUT TYPE=submit ONMOUSEOVER='window.status="Did you fill in all required fields?";' ONMOUSEOUT='window.status="";'>

When an event attribute is used, a default scripting language must be specified for the document by setting the Content-Script-Type HTTP header to the media type of the scripting language. The previous example could use the following META element in the document's HEAD:

<META HTTP-EQUIV="Content-Script-Type" CONTENT="text/javascript">

The common event attributes are device-dependent and largely tailored for the graphical user interface. The available events are as follows:

More Information

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