SOURCE

<div class="a">
  <div class="b">
    <div class="c"></div>
  </div>
</div>

.a {
  width: 200px;
  height: 200px;
  background: red;
}
.b {
  position: relative;
  width: 100px;
  height: 100px;
  background: blue;
  z-index: 0; /* 定位元素，且其z-index值不为auto才能构成层叠上下文 */
}
.c {
  position: absolute;
  width: 150px;
  height: 150px;
  background: yellow;
  z-index: -1;
}