为什么在 div1 和 div-a 下面出现​ ?
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
*{padding:0;margin:0;}
.div1{background:red;}
.div-a{height: 200px;}
.div-b{background:#ccc;}
</style>
</head>
<body>
<div class="div1">
<div class="div-a"></div>
<div class="div-b"></div>
</div>
</body>
</html>
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
*{padding:0;margin:0;}
.div1{background:red;}
.div-a{height: 200px;}
.div-b{background:#ccc;}
</style>
</head>
<body>
<div class="div1">
<div class="div-a"></div>
<div class="div-b"></div>
</div>
</body>
</html>