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<style>
.b {
	width: 100px;
	height: 100px;
	background-color: blue;
	margin-top: -30px;
}
.b-child {
	width: 50px;
	height: 50px;
	background: green;
	position: absolute;
	z-index: 6;
}
.a {
    position: absolute;
    z-index: 5;
    top: 0;
    left: 0;
    height: 100px;
    width: 100px;
    background-color: red;
}
</style>

<div class="b">
    <div class="b-child"></div>
</div>
<div class="a">
</div>