box-sizing DEMO

CSS

.box{
	width: 50%;
	margin: 50px auto;
	background: #e6e6e6;
	line-height: 100px;
}
.box div{
	background: #333;
	text-align: center;
	font-size: 200%;
	color: #fff;
}
#box-default{
	padding: 50px;
	border: 20px solid #B20000;
}
#box-sizing{
	-moz-box-sizing: border-box;
	box-sizing: border-box;
	padding: 50px;
	border: 20px solid #B20000;
}

HTML

width:50% default

	
lorem

width:50% box-sizing:border-box指定

	
lorem

width:50% default

	
lorem

width:50% default

width:50% box-sizing:border-box指定

width:50% default