css/css-ui/box-sizing-001.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS Basic User Interface Test: box-sizing:border-box and CSS2.1 10.3.3</title>
<link rel="author" title="Florian Rivoal" href="http://florian.rivoal.net/">
<link rel="help" href="https://drafts.csswg.org/css-ui-3/#box-sizing">
<link rel="help" href="http://www.w3.org/TR/CSS21/visudet.html#blockwidth">
<link rel="match" href="reference/box-sizing-001-ref.html">
<meta name="assert" content="When box-sizing is border-box, the content width, rather than the computed value of the width property,
                             is considered when checking whether a block is larger than its containing block.">
<style>
#cb {
	position: absolute;
	width: 100px;
	height: 100px;
}
#red {
	position:absolute;
	top: 25px;
	right:5px;
	width: 70px;
	height: 70px;
	background:red;
	z-index: -1;
}
#test {
	box-sizing: border-box;
	margin-top: 25px;
	margin-right: 5px;
	margin-left: auto; /* should resolve to 25px, but would resolve to 0 if the wrong interpretation of width is used in CSS2.1 10.3.3 */
	padding-left: 25px;
	padding-right: 25px;
	width: 70px;
	height: 70px;
	background:green;
}
</style>
<body>
  <p>Test passes if there is a <strong>filled green square</strong> and <strong>no red</strong>.</p>
  <div id="cb">
	  <div id="red"></div>
	  <div id="test"></div>
  </div>
</body>