SOURCE

<div class="test1">
  块级元素
</div>
<hr />
<div class="test2">
  <span>
    行内元素
  </span>
</div>
<hr />
<div class="test3">
  <div class="test4">
    Flex
  </div>
</div>

.test1 {
  background: red;
  height: 100px;
  width: 100px;
  margin: 0 auto;
}

.test2 {
  background: yellow;
  height: 100px;
  width: 100%;
  text-align: center;
}

.test3{
  background:#f0f0f0;
  height:200px;
  width:100%;
  position: relative;
}
.test4{
  background:#a0a0a0;
  height:100px;
  width:100px;
  position:absolute;
  left:0px;
  right:0px;
  margin: 0 auto;
}