JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

article, aside {
    float: left;
    padding: 5px;
}

article {
    width: 100%;
    width: -webkit-calc(100% - 50px);
    width: -moz-calc(100% - 50px);
    width: calc(100% - 50px);
    background: #cee;
}

aside {
    width: 50px;
    background: #ece;
}

section {
    padding: 25px;
    background: #eee;
    overflow: hidden;
}

* {
    box-sizing: border-box;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
}