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=secondp>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:
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.
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:
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:
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.
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:
<A HREF="mailto:email@example.com" TITLE="Feedback on HTML 4.0 Reference">firstname.lastname@example.org</A>
<A HREF="http://www-genome.wi.mit.edu/ftp/pub/software/WWW/cgi_docs.html" TITLE="CGI.pm - a Perl5 CGI Library">CGI.pm</A>
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"
<TITLE>Welcome - Bienvenue</TITLE>
<SPAN LANG=en>Welcome</SPAN> -
<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.
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.
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., ").
<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: