//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;

    width: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;

}