JSFiddle - React, Tailwind, and code Playground

HTML

<div id="a">
foo
<div id="b">
bar
</div>
</div>

CSS

#a {
    background: cyan;
    min-height: 500px;
    position: relative;
    padding: 5px;
}

#b {
    height: 100%;
    background: red;
    position: absolute;
    top: 25px;
    bottom: 0;
    left: 0;
    right: 0;
}