CSS子元素居中(父元素宽高已知，子元素未知)

<style>
.container{width:400px; height:400px; position:relative;}
.center{position:absolute; left:0; top:0; bottom:0; right:0; margin:auto; 
         width:50px; height:50px;  //宽高可以不写
}
</style>
<div class="container">
  <div class="center"></center>
</div>