div{
      width: 400px;
      height: 400px;
      position: relative;
      border: 1px solid #465468;
 }
 img{
      position: absolute;
      margin: auto;
      top: 0;
      left: 0;
      right: 0;
      bottom: 0;
 }

<div>
   <img src="mm.jpg">
</div>

.container{
      width: 500px;
      height: 400px;
      border: 2px solid #379;
      position: relative;
 }
 .inner{
      width: 480px;
      height: 380px;
      background-color: #746;
      position: absolute;
      top: 50%;
      left: 50%;
      margin-top: -190px; /*height的一半*/
      margin-left: -240px; /*width的一半*/
 }

<div class="container">
    <div class="inner"></div>
</div>

div{
    width: 300px;
    height: 300px;
    border: 3px solid #555;
    display: table-cell;
    vertical-align: middle;
    text-align: center;
}
img{
    vertical-align: middle;
}

<div>
    <img src="mm.jpg">
</div>

div{
    border: 3px solid #555;
    width: 300px;
    height: 200px;
    display: table-cell;
    vertical-align: middle;
    text-align: center;
}
span{
    vertical-align: middle;
}

<div>
    <span>这是放在span中的文字，通过外层div设置display: table-cell以及vertical-align: middle实现垂直居中。</span>
</div>

.container{
      width: 300px;
      height: 200px;
      border: 3px solid #546461;
      display: -webkit-flex;
      display: flex;
      -webkit-align-items: center;
      align-items: center;
      -webkit-justify-content: center;
      justify-content: center;
 }
 .inner{
      border: 3px solid #458761;
      padding: 20px;
 }

<div class="container">
    <div class="inner">
        我在容器中水平垂直居中
    </div>
</div>