JSFiddle - React, Tailwind, and code Playground

HTML

<div class='parent'>
    <div class='child_static'></div>
    <div class='child_absolute'></div>
</div>

CSS

.parent {
    position: relative;
}

.child_static {
    height: 20px;
    background: blue;
}

.child_absolute {
    position: absolute;
    height: 20px;
    width: 100%;
    background: green;
}