css下的背景透明效果！

<style type="text/css">
body{background:url(http://www.google.cn/intl/zh-CN/images/logo_cn.gif);position:relative;width:80%;margin:40px auto;font-size:12px; text-align:center}
p{margin:0;}
.alphaContainer{border:solid 1px #000;overflow:hidden;padding:20px; margin:0 auto; background: rgba(0, 0, 0, 0.4);}
.alphaContainer:after {content:".";display:block;height:0;clear:both;visibility:hidden;}
.alphaContainer2{border:1px solid #f00;overflow:hidden;padding:20px;margin:20px auto;background:rgba(0,0,0,0.4);width:360px;}
.content2{position:relative;z-index:2;background:#33FFFF;width:100%;border:1px solid #00FFFF;}
.content{position:relative;z-index:2;background:#fff;width:100%;float:left;border:solid 1px blue;}
.content p{line-height:23px;display:inline-block;margin:10px; text-align:left;}
</style>

<!--[if IE]>
<style type="text/css">
.alphaContainer{background:transparent;filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#40000000,endColorstr=#40000000);zoom: 1;);}
</style>
<![endif]-->

<div class="alphaContainer">
    <div class="content">
        <p>透明的效果</p>

        <p>效果是透明的</p>
    </div>
</div>
<div class="alphaContainer2">
    <div class="content2">
         <p>
                透明效果
              </p>
         <p>
           css下控制的透明效果
         </p>
    </div>
</div>