SOURCE

<div class="box">
  <div class="center"></div>
</div>

.box {
  background-color: #ccc;
  position: relative;
  height: 200px;//必须
}
.center {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 100px;
  height: 100px;
  background-color: #fff;
}