<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>