DIV在另一个DIV里面垂直居中，水平居中

HTML：

 <div class="parent">
        <div class="children">
            <div class="close">
                <img src="image/ico/icon-close.png" />
            </div>
        </div>
    </div>

CSS：

   <style type="text/css">
        .parent {
            width:800px;
            height:800px;
            background:#286921;
             position:relative;
        }
        .children {
            border:3px #fff solid;
            width:80px;
            height:80px;
            margin:auto;
            top: 0; left: 0; bottom: 0; right: 0; 
            position: absolute;  
        }
        .close {

        }
    </style>