<div id="Cond" style="width: 1366px; height: 768px; top: 0; left: 0; background: #b3b3b3;
         position: fixed; z-index: 99; display: none;filter:alpha(Opacity=50);-moz-opacity:0.5;opacity: 0.5;">

</div>

$("#Cond").css("display","block");//显示
$("#Cond").css("display","none");//隐藏