<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
*{
margin: 0;
padding: 0;
}
div{
background: blue;
width: 200px;
height: 200px;
/*设置padding或者border均可解决问题*/
/* padding-top: 10px; */
border-top: 1px solid #000;
}
p{
background: red;
width: 50px;
height: 50px;
margin-top: 30px;
}
</style>
</head>
<body>
<div>
<p></p>
</div>
</body>
</html>
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