1.开启定位的元素处于同一层级
2.下方元素会默认掩盖上方的元素
3.z-index: n;可以改变层级
4.子元素永远处于父元素上方
5.opacity属性开启透明 范围0-1
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style type="text/css">
.box1
{
width: 200px;
height: 200px;
background-color: red;
position: relative;
opacity: 0.5;
}
.box2
{
width: 200px;
height: 200px;
background-color: yellow;
position: absolute;
top: 100px;
left: 100px;
opacity: 0.5;
}
.box3
{
width: 200px;
height: 200px;
background-color: yellowgreen;
position: absolute;
top: 200px;
left: 200px;
opacity: 0.5;
}
</style>
</head>
<body>
<div class="box1"></div>
<div class="box2"></div>
<div class="box3"></div>
</body>
</html>