firefox/testing/web-platform/tests/css/css-flexbox/flexbox_margin-ref.html

<!DOCTYPE html>
<title>flexbox | margins</title>
<link rel="author" href="http://opera.com" title="Opera Software">
<style>
div {
	background: black;
	margin: 1em;
	height: 4em;
}
</style>

<div></div>