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 {
    display: flex;
    flex-direction: column;
    min-height: 5rem;
}
.flex-item {
    flex-grow: 1;
    background-color: steelblue;
}
.element {
    height: 100%;
    background-color: orange;
}