拾起来提升-------布局

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

<style>
.box{
  position:absolute;
  width:200px; 
  height:200px;
  top:50%;
  left:50%;
  margin-left:-100px;
  margin-top:-100px;
}
</style>

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

<style>
.box{
 position:fixed;


 width:50%;
 height:50%;
 left:50%;
 top:50%;
-webkit-transform: translateX(-50%) translateY(-50%);
}
</style>

<div style="float:left; width:100%; height:200px;">
   <div style="margin-left:200px; background:#000; height:200px"></div>
</div>
<div style="float:left; width:200px; height:200px; margin-left:-100%; background:yellow"></div>