JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="left"></div>
    <div class="right"></div>
</div>

CSS

.container {
    display: flex;
}
.container > div {
    border:1px solid black;
    height:10px;
}

.left {
    width: 100px;
}

.right {
    width: 100%;
}