This code makes sure that all nested containers are always the same height. This is done by assuring that all nested elements are the same height as the containing parrent div. See working example: https://jsfiddle.net/3wwh7ewp/
This effect is achieved due to the property align-items being set to stretch by default.
HTML
<div class="container">
<div style="background-color: red">
Some <br />
data <br />
to make<br />
a height <br />
</div>
<div style="background-color: blue">
Fewer <br />
lines <br />
</div>
</div>
CSS
.container {
display: flex;
align-items: stretch; // Default value
}