part of the containing xul binding (the hidden attribute is removed via JS):

   <xul:hbox pack="end">
       <xul:vbox pack="start">
         <xul:image anonid="alarm-image"
                    class="alarm-image"
                    hidden="true"/>
       </xul:vbox>
   </xul:hbox>


css file:

.alarm-image {
    list-style-image: url("chrome://calendar/skin/alarm_frames.png");
    -moz-binding: url(chrome://calendar/skin/alarm-image.xml#calendar-alarm-image);
    -moz-image-region: rect(0, 14px, 11px, 0);
    width: 14px;
    height: 11px;
}


alarm-image.xul:

<bindings id="alarm-image-bindings"
    xmlns="http://www.mozilla.org/xbl"
    xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
    xmlns:xbl="http://www.mozilla.org/xbl">

  <binding id="calendar-alarm-image">
    <implementation>
      <field name="frames">6</field>
      <field name="timeout">200</field>
    </implementation>
  </binding>
</bindings>