SOURCE

<div class="blue"></div>


<div class="red-outer">
    <div class="red-inner">
        red inner
    </div>
</div>

*{
	margin: 0;
	padding: 0;
}

.blue {
    background: blue;
    height: 50px;
   	margin-top: 20px;
	margin-bottom: 20px;
}

.red-outer {
    background-color: red;
    height: 40px;
    margin-top: 20px;
    /* display:flow-root; */
    .red-inner {
        background-color: pink;
        margin-top: 20px;
    }
}