SOURCE

<div id="parent">
	<div id="child" style="height: 300px;"></div>
	<div id="child"></div>
</div>

#parent {
    display: flex;
    justify-content: center;
    align-content: center;
    /*  stretch让盒子内的每个元素的高度都等于行高 */
    align-items: stretch;
    width: 300px;
}

#child {
    flex: 1;
    outline: solid 1px;
}