透明背景的实现

<!-- S 浮层 -->
<div id="cancelShowDivBack" style="z-index:98;position:absolute;display:none;background:gray;filter:progid:DXImageTransform.Microsoft.Alpha(opacity=40,finishOpacity=80,style=2);opacity:0.5">
 </div>
<div class="box layer layerS omLayer" style="z-index:299;top:500px;left:400px;bottom:300px;position:absolute;display:none;" id="cancelShowDiv">
    	<div class="hd">
    		<h3><a title="温馨提示" href="javascript:void(0)">温馨提示</a></h3>
    		<a class="closeLayer" title="关闭" href="javascript:void(0)" onclick="closeCancel()">关闭</a>
    	</div>
    	<div class="bd">
    		<p>失效后此优惠券将不能继续领取，但不影响已领取的优惠券的使用，请你及时删除推广页面的的代码，确认要失效此优惠券吗？</p>
    		<div class="btn_box"><a class="btn" href="javascript:void(0)" onclick="sureCancelShopCoupon('$lSellerId','$strSellerTypeName','3')"><span>确 定</span></a><a class="btn" href="javascript:void(0)" onclick="closeCancel()"><span>取 消</span></a></div>
    	</div>
    </div>
	<div class="box layer layerS omLayer" style="z-index:299;top:500px;left:400px;bottom:300px;position:absolute;display:none;" id="copyShowDiv">
    	<div class="hd">
    		<h3><a title="温馨提示" href="javascript:void(0)">温馨提示:请将以下代码复制并粘贴到你需要的页面</a></h3>
    		<a class="closeLayer" title="关闭" href="javascript:void(0)" onclick="closeCancel()">关闭</a>
    	</div>
    	<textarea id="copyShowContent"  cols="51" rows="10" style="width:393px;">
        </textarea>
    </div>

<!-- E 浮层 -->

//是否失效优惠券
function cancelShopCoupon(lActivityId) {
	$("#lActivityId").val(lActivityId);
	var height = $(window).height(); // 浏览器当前窗口可视区域高度
	var width = $(window).width();// 浏览器当前窗口可视区域宽度
	var cancelShowDivBack = document.getElementById("cancelShowDivBack");
	var cancelShowDiv =document.getElementById("cancelShowDiv");
	var heightTemp=$(document).scrollTop();
	document.body.style.overflow="hidden";
	cancelShowDivBack.style.display="block";
	cancelShowDiv.style.display="block";
	cancelShowDivBack.style.zIndex=998;
	cancelShowDiv.style.zIndex=1000;
	cancelShowDivBack.style.top="0px";
	cancelShowDivBack.style.left="0px";
	var selW=width/2-(cancelShowDiv.offsetWidth/2)+"px";
	var selH=height/2+heightTemp/2+"px";
	cancelShowDiv.style.top=selH;
	cancelShowDiv.style.left=selW;
	//showAlertMeesageForCoupon(cancelShowDiv.style.top+":"+cancelShowDiv.style.left);
	cancelShowDivBack.style.width=width+"px";
	cancelShowDivBack.style.height=2*height+"px";
}