JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
    <div class="child">
        100% height
        (parent is 200px)
    </div>
</div>

<div class="viewport-height">
    100vh height
</div>

CSS

body,
html {
    height: 100%;
}

.parent {
    background: lightblue;
    float: left;
    height: 200px;
    padding: 10px;
    width: 50px;
}

.child {
    background: pink;
    height: 100%;
    width: 100%;
}

.viewport-height {
    background: gray;
    float: right;
    height: 100vh;
    width: 50px;
}