Pure CSS aspect ratio with no spacer images or js! :)

dabblet.css

/*
 * Pure CSS aspect ratio with no spacer images or js! :)
 */

body {
	width: 36%;
	margin: 8px auto;
}

div.stretchy-wrapper {
	width: 100%;
	padding-bottom: 56.25%; /* 16:9 */
	position: relative;

	background: blue;
}

div.stretchy-wrapper > div {
	position: absolute;
	top: 0; bottom: 0; left: 0; right: 0;

	color: white;
	font-size: 24px;
	text-align: center;
}

/* Other aspect ratios to try:
 * 56.25% = 16:9
 * 75% = 4:3
 * 66.66% = 3:2
 * 62.5% = 8:5
 */

dabblet.html

<div class="stretchy-wrapper"><div>Resize your browser</div></div>

settings.json

{"view":"split","fontsize":"100","seethrough":"","prefixfree":"1","page":"css"}