em size

dabblet.css

/* em size */

div {
	width: 1em;
	height: 1em;
	background: #f33;
	padding: 0;
	margin: 0;
	border: 0;
}

dabblet.html

<meta name="viewport" content="width=device-width,initial-scale=1">
<div></div>
<script>
	document.write(getComputedStyle(document.querySelector('div'),"").height);
</script>

settings.json

{"view":"split","prefixfree":"","page":"css"}