透明遮罩层

<input type="button" value="订阅 " class="bookbtn" id="bookmailtop1">

<dl class="pop_dl" id="pol_dl_mail" >
  <dt><span>关闭</span></dt>
  <dd>
    <ul class="bmail_ok">
      <li>
        <h4>邮件订阅成功！</h4>
        <span>优惠信息将每天及时发送到您的邮箱。</span></li>
      <li>• 收不到邮件？有可能被误判为垃圾邮件了，请到垃圾邮件文件夹找找。</li>
      <li>• 如不想继续收每日提醒邮件，您可以在邮件中随时取消订阅。</li>
    </ul>
  </dd>
</dl>
<div id="popbg" ></div>

/* CSS 遮罩弹窗 */
#popbg {BACKGROUND: #000; FILTER: alpha(opacity=50); LEFT: 0px; WIDTH: 100%; POSITION: absolute; TOP: 0px; HEIGHT: 100%; opacity: 0.5; z-index:9999998;display:none;}  
 
.pop_dl {margin:0 auto;POSITION: absolute; z-index:9999999;display:none; border:1px solid #decea9; background-color:#fff; width:400px;} 
.pop_dl dt { border-bottom:1px solid #decea9; height:39px;text-align:right; padding-right:10px; font-size:14px;background:url(close.gif) no-repeat 374px 12px;background-color:#f7e9cf; }
.pop_dl dt span { display:block; height:16px; line-height:16px;  cursor:pointer; width:44px; float:right; text-align:left; margin-top:11px;* margin-top:13px !important;*margin-top:13px;}
.pop_dl dt div { display:block; height:16px; line-height:16px;  cursor:pointer; width:44px; float:right; text-align:left; margin-top:11px;* margin-top:13px !important;*margin-top:13px;}
.pop_dl dt b { float:left;font-family:arial,sans-serif; font-size:18px; line-height:20px; margin-top:11px;* margin-top:13px !important;*margin-top:13px; font-weight:normal; margin-left:15px;}
.pop_dl dd { padding:15px;}


.bmail_ok {background:url(ok_bg.gif) no-repeat 0 0; color:#C90809;  margin:10px 25px;padding-left:70px;}
.bmail_ok li { margin-bottom:10px; padding-top:10px;line-height:18px;}
.bmail_ok h4 {font-family:simhei,arial,sans-serif; font-size:18px; }
.bmail_ok span { color:#666;}

<script type="text/javascript">

//弹出对话框关闭操作
$('.pop_dl>dt>span').click(function(){
    $("#popbg").fadeOut(); 
    $('.pop_dl').fadeOut(); 
});
$('#bookmailtop1').click(function(){
popdiv("#pol_dl_mail","400","auto",0.2);
});

function popdiv(popdiv,width,height,alpha){
    var A = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop || 0; 
    var D = 0;
    D = Math.min(document.body.clientHeight, document.documentElement.clientHeight);
    if (D == 0) {
    D = Math.max(document.body.clientHeight, document.documentElement.clientHeight)
    } 
    var topheight = (A + (D - 300) / 2)-100 + "px"; 
    alert(A);alert(D);alert(topheight);
    $("#popbg").css({height:$(document).height(),filter:"alpha(opacity="+alpha+")",opacity:alpha})
    $("#popbg").fadeIn();
    //$(popdiv).removeClass();
    //$(popdiv).attr("class","pop_out ");
    $(popdiv).css({left:(($(document).width())/2-(parseInt(width)/2))+"px",top:topheight});
    $(popdiv).fadeIn();
}
</script>