<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
div {
/* border 撑大盒子尺寸 */
/* 盒子尺寸 = width / height + 边框线 */
/* 如果400 * 400 尺寸 */
width: 380px;
height: 380px;
background-color: green;
border: 10px solid #000;
}
</style>
</head>
<body>
<div>div</div>
</body>
</html>