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dabblet.css

.calc-emulator {
	width: 600px;
	border-left: 200px solid transparent;
	box-sizing: border-box;
	
	/*visual*/
	background: skyblue;
	height: 280px;
	}
	
.sidebar {
	margin-left: -200px;	
	width: 200px;
	
	float: left;
	
	/*visual*/
	height: 200px;
	background: #222;
	color: white
	}
	
.content {
	width: 100%;
	
	/*visual*/
	height: 200px;
	background:orange;
	}

dabblet.html

<div class="calc-emulator">
	<div class="sidebar">
	200px / <br/>margin-left: -200px
	</div>

	<div class="content">
	width: 100% - 200px
	</div>
	width: 600px, with a transparent left-border(width of .sidebar)
</div>

settings.json

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