w3cschool地址：http://www.w3school.com.cn/cssref/pr_box-sizing.asp
box-sizing有三个值：
- content-box (default)，border和padding不计算入width和height之内
- border-box，border和padding都计算入width和height之内
- inherit，继承父元素的box-sizing属性
举个栗子，代码如下
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>box-sizing</title>
<style>
.content-box {
width: 50px;
height: 50px;
border: solid 50px red;
padding: 50px;
background-color: yellow;
box-sizing: content-box;
}
.border-box {
width: 50px;
height: 50px;
border: solid 50px red;
padding: 50px;
background-color: yellow;
box-sizing: border-box;
}
</style>
</head>
<body>
<div class="content-box"></div>
<br/>
<div class="border-box"></div>
</body>
</html>
页面效果：
