JSFiddle - React, Tailwind, and code Playground

HTML

<div id="parent">
    <div id="child"></div>
</div>

CSS

html, body {
    padding: 0;
    margin: 0;
    height: 100%;
}

#parent {
    background-color: yellow;
    height: 100%;
    padding: 10px;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

#child {
    background-color: blue;
    height: 20%;
    width: 100%;
    margin-bottom:-10%;
    
}