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7.5. Definition Lists

HTML and XHTML also support a list style entirely different from the ordered and unordered lists we've discussed so far: definition lists. Like the entries you find in a dictionary or encyclopedia, complete with text, pictures, and other multimedia elements, the definition list is the ideal way to present a glossary, list of terms, or other name/value lists.

7.5.1. The <dl> Tag

The definition list is enclosed by the <dl> and </dl> tags. Within the tags, each item in a definition list is composed of two parts: a term followed by its definition or explanation. Instead of <li>, each item name in a <dl> list is marked with the <dt> tag, followed by the item's definition or explanation as it is marked by the <dd> tag.

Unless you change the display attributes with style sheet rules, browsers typically render the item or term name at the left margin and render the definition or explanation below it and indented. If the definition terms are very short (typically less than three characters), the browser may choose to place the first portion of the definition on the same line as the term. See how the source XHTML definition list below gets displayed by Netscape in Figure 7-8:

<h3>Common Kumquat Parasites</h3>
<dl>
  <dt>Leaf mites</dt>
  <dd>The leaf mite will ravage the Kumquat tree, stripping it
      of any and all vegetation.</dd>
  <dt>Trunk dropsy</dt>
  <dd>This microscopic larvae of the common opossum 
      chigger will consume the structural elements of the
      tree trunk, causing it to collapse inward.</dd>
</dl>
Figure 7-8

Figure 7-8. A definition list as presented by Netscape

As with other list types, you can add more space between the definition list items by inserting paragraph <p> tags at the end of their content or by defining a spacious style for the respective tags.

<dl>

Function:

Define a definition list

Attributes:

CLASS

ONKEYUP

COMPACT

ONMOUSEDOWN

DIR

ONMOUSEMOVE

ID

ONMOUSEOUT

LANG

ONMOUSEOVER

ONCLICK

ONMOUSEUP

ONDBLCLICK

STYLE

ONKEYDOWN

TITLE

ONKEYPRESS

EndTag:

</dl>; never omitted

Contains:

dl_content

Used in:

block

7.5.2. The <dt> Tag

This <dt> tag defines the term component of a definition list. It is valid only when used within a definition <dl> list preceding the term or item, before the <dd> tag and the term's definition or explanation.

<dt>

Function:

Define a definition list term

Attributes:

CLASS

ONKEYUP

DIR

ONMOUSEDOWN

ID

ONMOUSEMOVE

LANG

ONMOUSEOUT

ONCLICK

ONMOUSEOVER

ONDBLCLICK

ONMOUSEUP

ONKEYDOWN

STYLE

ONKEYPRESS

TITLE

End tag:

</dt>; may be omitted in HTML

Contains:

text

Used in:

dl_content

Traditionally, the definition term that follows the <dt> tag is short and sweet -- a word or few. Technically, it can be any length. If long, the browser may exercise the option of extending the item beyond the display window, or wrap it onto the next line where the definition begins.

Since the end of the <dt> tag immediately precedes the start of the matching <dd> tag, it is unambiguous and so not required. However, the XHTML standard insists that it be present. So get used to including it in your documents.

7.5.3. The <dd> Tag

The <dd> tag marks the start of the definition portion of an item in a definition list. According to the HTML and XHTML standards, <dd> belongs only inside a definition <dl> list, immediately following the <dt> tag and term and preceding the definition or explanation.

<dd>

Function:

Define a definition list term

Attributes:

CLASS

ONKEYUP

DIR

ONMOUSEDOWN

ID

ONMOUSEMOVE

LANG

ONMOUSEOUT

ONCLICK

ONMOUSEOVER

ONDBLCLICK

ONMOUSEUP

ONKEYDOWN

STYLE

ONKEYPRESS

TITLE

End tag:

</dd>; always omitted in HTML

Contains:

flow

Used in:

dl_content

The content that follows the <dd> tag may be any HTML construct, including other lists, block text, and multimedia elements. Although treating it otherwise identically as conventional content, browsers typically indent definition list <dd> definitions. And since the start of another term and definition (<dt>) or the required end tag of the definition (</dl>) unambiguously terminates the preceding definition, the </dd> tag is not needed and its absence makes your source text more readable. However, and once again, XHTML insists that the end tag appear in your documents, so you may as well get used to adding </dd> to your documents.



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