1. div标签未定义父层大小时父层自适应子层大小

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        *{
            padding: 0;
            margin:0;
        }
        #main{
        }
        #content1{
            width:170px;
            height:190px;
            background-color:red;
        }
        #content2{
            background-color:green;
            width:170px;
            height:190px;
        }
    </style>
</head>
<body>
    <div id="main">
        这是父层
        <div id="content1">这是子层1</div>
        <div id="content2">这是子层2</div>
    </div>
</body>
</html>

可以看到未设置main样式的，父容器的大小会自动根据设置好大小的子容器来设定

2. div标签定义父层大小且子层大小超出父层大小时

<style>
        *{
            padding: 0;
            margin:0;
        }
        #main{
            width:170px;
            height:190px;
            margin: 50px auto;
            background-color: yellow;
        }
        #content1{
            width:170px;
            height:190px;
            background: rgba(0,0,0,0.3);
        }
        #content2{
            width:170px;
            height:190px;
            background: rgba(1,1,1,0.9);
            color: white;
        }
    </style>