//已知宽高的情况
.div1_container{
border:1px solid #00ee00;
height:300px;
position:relative;
}
.div1_center{
position:absolute;
top:50%;
left:50%;
margin:-100px 0 0 -100px;
200px;
height:200px;
border:1px solid red;
}
//未知宽高的情况
.div2_container{
text-align:center;
height:400px;
border:1px solid #0000ed;
}
.div2_container:before{
content:'';
height:100%;
display:inline-block;
vertical-align:middle;
}
.div2_center{
display:inline-block;
vertical-align:center;
border:1px solid red;
}