Appearance
无论 content-box 还是 border-box， content、padding、border都会被填充（border要设 置为透明才能看得见, 否则会覆盖）
html
<style>
#box1 {
box-sizing: border-box;
width: 200px;
height: 200px;
padding: 20px;
border: transparent 20px solid;
margin: 20px;
background-color: red;
}
</style>
</head>
<body>
<div id="box1"></div>