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实现重点：背景div设置属性position: relative;，遮罩层div设置属性position: absolute;，背景属性background，属性bottom: 0;让遮罩层div在背景div里面。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style>
.bacimg{
width: 400px;
height: 300px;
position: relative;
}
.bacimg img{
width: 400px;
height: 300px;
}
.cover{
width: 100%;
position: absolute;
background: rgba(0,0,0,0.6);
color: #fff;
opacity: 0.6;
bottom: 0;
}
</style>
</head>
<body>
<div class="bacimg">
<img src="../../img/ad2.jpg" />
<div class="cover">
aaaa
</div>
</div>
</body>
</html>
结果图：
