JSFiddle - React, Tailwind, and code Playground

HTML

<div id="c">
    <div id="a">
        <div id="b"></div>
    </div>
</div>

CSS

#c{
    background:blue;
    width: 200px;
    height: 200px;
    padding: 20px;
}
#a{
    height: 100%;
    background:#ffffff;
    position: relative;
}
#b{
    background:green;
    width: 100%;
    height: 20px;
    position: absolute;
}