EXAMPLE
Run 
<!DOCTYPE html>
<html>

<head>
	<title>Box-sizing property in CSS</title>
	<style type="text/css">
		div {
		  width: 160px;
		  height: 80px;
		  padding: 20px;
		  border: 8px solid red;
		  background: gray;
		}
		
		.content-box {
		  box-sizing: border-box;
		width: 100%;
		border: solid #5B6DCD 10px;
		padding: 5px;
		 
		}
	</style>
</head>

<body>
	<div class="content-box">Content box</div>
	<br>
	<div class="border-box">Border box</div>
</body>

</html>