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dabblet.css

div { transition:all 0.5s; }
.container{width:200px;}

.box { opacity:0.666667; width: 200px; height: 200px; background: red; margin-bottom: 20px; cursor: pointer;}

.container:hover .box:hover {
	opacity:1;

}
.container:hover .box:not(:hover) {
  opacity: .333333;

}

dabblet.html

<div class="container">
	<div class="box">sample</div>
	<div class="box">sample</div>	
	<div class="box">sample</div>		
</div>

settings.json

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