div背景透明样式：
样式代码：
.alert{filter:alpha(opacity=100); /* IE */ -moz-opacity:1.0; /* Moz + FF */
opacity: 1.0; height:300px; width:500px; background:#ccc; left:50%; top:50%;margin-top:-150px;margin-left:-250px;position:absolute;z-index:99; text-align:center; padding:20px;}
使用方法：
<div class=”alert”>
背景透明
</div>
html代码：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-
equiv="Content-Type" content="text/html; charset=utf-8" />
<title>100素材网</title>
<style type="text/css">
.alert
{filter:alpha(opacity=100); /* IE */ -moz-opacity:1.0; /* Moz + FF */ opacity: 1.0; height:300px; width:500px; background:#ccc; left:50%; top:50%;margin-top:-150px;margin-left:
-250px;position:absolute;z-index:99; text-align:center; padding:20px}
</style>
</head>
<body>
<div class="alert">100素材网</div>
</body>
</html>