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dabblet.css

body {
	padding-top: 100px;
}
.container {
    float:left;
	height: 300px;
	width: 100px;
	border: 1px solid #000;
	margin-right: 100px;
}
.box {
	width: 100%;
	height: 100%;
	opacity: 0.5;
}
.box_1 {
	background-color: red;
	/* 100% of width =/ */
	margin-top: -100%;
}
.box_2 {
	background-color: blue;
}
.box_3 {
	background-color: green;
}

dabblet.html

<div class="container">
	<div class="box box_1"></div>
	<div class="box box_2"></div>
	<div class="box box_3"></div>
</div>

settings.json

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