user/src/com/google/gwt/user/client/ui/NotificationMole.ui.xml

<!DOCTYPE ui:UiBinder SYSTEM "http://dl.google.com/gwt/DTD/xhtml.ent">
<ui:UiBinder xmlns:ui='urn:ui:com.google.gwt.uibinder'
  xmlns:g='urn:import:com.google.gwt.user.client.ui'>
  <ui:style>
    .container {
      position: absolute;
      height: 0;
      text-align: center;
      width: 100%;
    }

    .centered {
      margin-left: auto;
      margin-right: auto;
      border-left: 1px solid #96a2b5;
      border-right: 1px solid #96a2b5;
      border-bottom: 1px solid #96a2b5;
      background-color: #e5edf9;
      padding: 5px;
      overflow: hidden;
      display: inline-block;
    }

    .notificationText {
      font-family: Helvetica;
      font-size: 1em;
    }
  </ui:style>
  <g:HTMLPanel styleName='{style.container}'>
    <div class='{style.centered}' style='display:none' ui:field='borderElement'>
      <div ui:field='heightMeasure'>
        <span class='{style.notificationText}' ui:field='notificationText'></span>
      </div>
    </div>
  </g:HTMLPanel>
</ui:UiBinder>