SOURCE

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

.c {
  position: relative;  /* 去掉这个设置后，看蓝色边框的变化 */
}
.a {
  position: absolute;
  width: 100%;
  height: 100%;
  border: 1px solid blue;
}
.b {
  width: 100px;
  height: 100px;
  background: red;
}