Box Sizing

dabblet.css

/**
 * Box Sizing
 */
body {
	background: #fff;
	font: 19px/1.3 "Helvetica Neue", sans-serif;
}
p {
	display:block;
	margin:20px;
	width: 280px;
	padding:20px;
	background-color:#f0f0f0;
	position:relative;
}
#border {
	box-sizing:border-box;
}
#content {
	box-sizing:content-box;
}
#border:before, #content:before {
	content:"border";
	position:absolute;
	top:0;
	left:0;
	padding:5px;
	font-size:14px;
	color:#fff;
	background-color:rgba(51,51,102,0.5);
}
#border:before {
	content:"border";
}
#content:before {
	content:"content";
}

dabblet.html

<p id="border">Curso de HTML5 e CSS3 na e-Genial</p>
<p id="content">Curso de HTML5 e CSS3 na e-Genial</p>

settings.json

{"view":"separate","seethrough":"","prefixfree":"1","page":"all"}