plain</tt>, <tt>image/jpeg</tt>.  An icon
    can be associated to a base type by using a <tt>*</tt> as the
    sub-type.  Examples: <tt>text/*</tt>, <tt>image/*</tt>.  The
    default icon can be specified by using a content-type of
    <tt>*/*</tt>.
    </p>
    </dd>

<dt><var>url</var></dt>
<dd><p>The URL of the icon.  Example: <tt>http://host/icons/text.gif</tt>,
    <tt>/icons/image.gif</tt>.
    </p>
    <p>The url can be prefixed with a
    width and height specification to provide rendering hints t