html绝对居中

html绝对居中，移动端极其好用
<div class="db">
<div class="dm"></div>
</div>.db{
width:400px;
height:200px;
background:#ccc;
}
.dm{
width:50px;
height:50px;
background:pink;
position:absolute;
left:0;right:0;
top:0;bottom:0;
margin:auto;
}