BOX_SIZ
Run 
<!DOCTYPE html>
<html>
<head>
	<title>CSS Box Sizing</title>
	<style> 
		.box1 {
		  width: 300px;
		  height: 100px;
		  border: 2px solid blue;
		  padding: 40px;
		  box-sizing: content-box;
		  margin-bottom: 30px;
		}

		.box2 {
		  width: 300px;
		  height: 100px;  
		  border: 2px solid blue;
		  padding: 40px;
  		  box-sizing: border-box;
		}
	</style>
</head>
<body>
	<div class="box1">This div element has width:300px; and height:100px;.</div>
	<div class="box2">This div element has width:300px; and height;100px;.</div>
</body>
</html>