JSFiddle - React, Tailwind, and code Playground

HTML

<div id='square'>
    <div id='rectangle' /></div>

CSS

#square {
    position:relative;
    height:100px;
    width:100px;
    background:red;
}
#rectangle {
    position:absolute;
    width:50px;
    background:blue;
    height:100%;
}