width: 500px;
height: 250px;
padding: 20px;
border: 20px #000 solid;
margin: 20px;
box-sizing: （指定なし）

width: 500px;
height: 250px;
padding: 20px;
border: 20px #000 solid;
margin: 20px;
box-sizing: content-box;

width: 500px;
height: 250px;
padding: 20px;
border: 20px #000 solid;
margin: 20px;
box-sizing: border-box;