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dabblet.css

.boxA {
	background: -moz-element(#boxB);
	width: 100px;
	height: 100px;
	display: table;
	text-align: center;
	border: 1px solid red;
}

.boxA  div{
	display: table-cell;
	vertical-align: middle;
}

.boxB {
	background: -moz-element(#boxA);
	width: 100px;
	height: 100px;
	display: table;
	text-align: center;
	border: 1px solid pink;
}

.boxB div {
	display: table-cell;
	vertical-align: middle;
}

dabblet.html

<div class="boxA" id="boxA">
	<div>BOX A</div>
</div>


<div class="boxB" id="boxB">
    <div>
	BOX B
	</div>
</div>

settings.json

{"view":"split-vertical","prefixfree":"1","page":"css"}