css让div水平垂直居中

.div1{
    width:200px;
    height:300px;
    border:1px solid #000;
    position: relative;
}
.div2{
    width: 40px;
    height:40px;
    background:green;
    position: absolute;
    margin: auto;
    top:0;
    left:0;
    right:0;
    bottom:0;
}

<div class="div1">
    <div class="div2">居中</div>
</div>

.div3{
    width:200px;
    height:300px;
    border:1px solid #000;
    display: table-cell;
    vertical-align: middle;
}
.div4{
    width: 40px;
    margin: 0 auto;
}

<div class="div3">
    <div class="div4">居中</div>
</div>