div或img图片高度随宽度自适应的方法

<div class="box">
 <span>行内元素垂直居中</span>
 <div class="img-box">
  <img src="123.jpg"/>
        </div>
</div>

.box{
 width: 50%;
 margin: 50px auto;
}
.img-box{
 width: 100%;
 position:relative;
 z-index:1;
}
.img-box img{
 position:absolute;
 top:0;
 bottom:0;
 left:0;
 right:0;
 width:100%;
 margin:auto;
 z-index: -1;
 *zoom:1;
}
.img-box:before {
 content: "";
 display: inline-block;
 padding-bottom: 100%;
 width: 0.1px; /*必须要有数值，否则无法把高度撑起来*/
 vertical-align: middle;
}

<div class="img-box">
 <img src="123.jpg"/>
</div>

.img-box{
 padding-bottom:100%;
}
.img-box img{
 position:absolute;
 top:0;
 bottom:0;
 left:0;
 right:0;
 width:100%;
 margin:auto;
}