居中布局（水平+垂直）

<div class="parent">
    <div class="child"></div>
</div>

.parent{
   width:1000px;
   height:500px;
   background:#ccc;
   
   display:table-cell;
   vertical-align:middle;
}

.child{
   width:200px;
   height:200px;
   background:#c9394a;

   display:table; /*block*/
   margin:0 auto;
}

.parent{
   width:1000px;
   height:500px;
   background:#ccc;

   position:relative;   
}

.child{
   width:200px;
   height:200px;
   background:#c9394a;

   position:absolute;
   top:50%;
   left:50%;
   transform:translate(-50%,-50%);
}