SOURCE

<div class='outer'>
	<div class='inner'></div>
	<div class='inner2'></div>
</div>

.outer {
    border: 1px solid green;
    /* display: flex;
    justify-content: center;
    align-items: center; */
    position: relative;
    display:flow-root;
}

.inner,
.inner2 {
    box-sizing: border-box;
    width: 100px;
    height: 100px;
    background: red;
}
.inner{
    float: left;
    margin-right:20px;
}
.inner2 {
    background: blue;
    /* float: left; */
    display:flow-root;
}