Why "tabs-icon-top" doesn't work


#1
<div class="bar bar-footer">
      <div class="tabs tabs-icon-top">
          <a class="tab-item"><i class="ion ion-navigate"></i>导航 </a>
          <a class="tab-item"><i class="ion ion-camera"></i>拍照 </a>
          <a class="tab-item"><i class="ion ion-ios7-telephone"></i>热线 </a>
          <a class="tab-item"><i class="ion ion-gear-a"></i>设置 </a>
      </div>