<div id="login_area">
               <div class="loginCon">
                 <div id="close"></div>
               </div>
             </div>
    //获取可视区域
    var wHeight = document.documentElement.clientHeight;
    var wWidth = document.documentElement.clientWidth;

     //登录框设置
    var ologin_area = document.getElementById("login_area");
    //获取ologin_area的宽度和高度
    var dHeight= ologin_area.offsetHeight;
    var dWidth= ologin_area.offsetWidth;
		ologin_area.style.top= wHeight/2-dHeight/2+"px";
		ologin_area.style.left=wWidth/2-dWidth/2+"px";

好像没有减去登录框的高宽，登录框只能显示在遮罩层的右下角