不定宽高的水平垂直居中---transform

            position: absolute;
            left:50%;
            top:50%;
            transform: translate(-50%,-50%);
            -moz-transform: translate(-50%,-50%);
            -ms-transform: translate(-50%,-50%);
            -o-transform: translate(-50%,-50%);
            -ms-transform: translate(-50%,-50%);