弹出框——边缘透明

CSS部分：
p{margin:0;padding:0;}
.alertbox{position:relative;width:500px;height:auto;overflow:hidden;padding:5px;border-radius:5px;-webkit-border-radius:5px; -moz-border-radius:5px;}
.alertbox_hd{background:#F8F8F8;border-top-right-radius:3px;border-top-left-radius:3px;-moz-radius-topright:3px;-moz-radius-topleft:3px;height:24px;line-height:24px;padding:0 10px;}
.alertbox_bd{background:#fff;padding:10px;}
.alert_blank{width:100%;height:2000px;position:absolute;left:0;top:0;background:#333;opacity:0.3;filter:Alpha(Opacity=30);z-index:-5;}

BODY部分：
<div class="alertbox">
<div class="alertbox_hd">sd</div>
<div class="alertbox_bd">
<p>sdfd</p>
<p>sdfd</p>
<p>sdfd</p>
<p>sdfd</p>
<p>sdfd</p>
</div>
<div class="alert_blank"></div>
</div>