SOURCE

<div class="one">
  <div class="two">
    two
  </div>
  <div class="three">
    three
  </div>
</div>
<div class="four">
  <div class="five">
    five
  </div>
  <div class="six">
    six
  </div>
</div>

<p style="margin-top:30px;">堆叠顺序：.three .two .one .five .six .four</p>

.one {
  position: relative;
  z-index: 2;
  .two {
    z-index: 6;
  }
  .three {
    position: absolute;
    z-index: 5;
  }
}

.four {
  position: absolute;
  z-index: 1;
  .five {}
  .six {
    position: absolute;
    top: 0;
    left: 0;
    z-index: -1;
  }
}