JSFiddle - React, Tailwind, and code Playground

HTML

<div class="flex">
    <div class="flex-item">
        <div class="element">
            The orange element should be as tall as the blue element
        </div>
    </div>
</div>

CSS

.flex {
    height: 100px;
    display: -webkit-flex;
    display: flex;
    flex-direction: column;
}
.flex-item {
    background-color: steelblue;
    -webkit-flex: 1 0 0;
    flex: 1 0 0;
}
.element {
    height: 100%;
    background-color: orange;
}