JSFiddle - React, Tailwind, and code Playground

HTML

<div id="left"></div>
<div id="right"></div>

CSS

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

#left{
    height: 100%;
    width: calc(100% - 100px);
    background-color: red;
    float: left;
}

#right{
    height: 100%;
    width: 100px;
    background-color: black;
    float: right;
}