Untitled

dabblet.css

*, *:after, *:before {
	margin: 0;
	padding: 0;
	box-sizing: border-box;
	position: relative;
	}
	html, body {
		height: 100%;
		}
	
	body {
		background: url(http://www.lorempixel.com/600/500) top left fixed no-repeat;
		background-size: cover;
		}
	.first, .second {
		position: absolute;
		top: 100%;
		left: calc(50% - 300px);
		
		width: 600px;
		height: 100%;
		
		background: url(http://www.lorempixel.com/600/500/city) top left no-repeat;
		background-size: cover;
		
		overflow: auto;
		}
	.second {
		background: url(http://www.lorempixel.com/600/500/city/2);
		}

dabblet.html

<div class="first">
	<div class="second"></div>
</div>

settings.json

{"view":"separate","fontsize":"100","seethrough":"","prefixfree":"1","page":"all"}