演示代码：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style type="text/css">
div{
width: 220px;height: 60px;line-height: 60px;
position: relative;
/* background-color: red; */
/* background-color: #ff0000; */
background-color: rgb(255,0,0);
/* background-color: rgba(255,0,0,0.2); */
}
p{
width: 100px;height: 30px;background-color: blue;
position: absolute;top: 0;left: 0;
background-color: rgba(0,0,255,0.2);
}
</style>
</head>
<body>
<!--
背景颜色：
background-color: 颜色值;
如果没有设置背景颜色，那么默认透明，默认值
-->
<div>
我要自学
<p></p>
</div>
</body>
</html>
