<style>
.top {
	border: 1px solid green;
	width: 200px;
	overflow: hidden;
}

.main {
	width: 300px;
}

.nested {
	border: 1px solid red;
	display: block;
	height: 50%;
	width: 200px;
}

</style>

<div class=top><div class=main><div class=nested>This nested div has percent height value and the closest block level containing block box is the ICB.</div></div></div>

<div class=top style="height: 200px"><div class=main><div class=nested>This nested div has percent height value and the closest block level containing block box is 2 containing blocks away.</div></div></div>