JSFiddle - React, Tailwind, and code Playground

HTML

<section>
    <aside>I'm 50px width!</aside>
    <article>I fill out all the available space!</article>
</section>

CSS

article, aside {
    padding: 5px;
}
article {
    overflow: hidden;
    background: #cee;
}
aside {
    width: 50px;
    background: #ece;
    float:right;
}
section {
    padding: 25px;
    background: #eee;
    overflow: hidden;
}
* {
    box-sizing: border-box;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
}