堆叠效果

把多个进度条放入同一个 .progress 中，使它们呈现堆叠的效果。

<div class="progress">
  <div class="progress-bar progress-bar-success" style="width: 35%">
    <span class="sr-only">35% Complete (success)</span>
  </div>
  <div class="progress-bar progress-bar-warning progress-bar-striped" style="width: 20%">
    <span class="sr-only">20% Complete (warning)</span>
  </div>
  <div class="progress-bar progress-bar-danger" style="width: 10%">
    <span class="sr-only">10% Complete (danger)</span>
  </div>
</div>