JSFiddle - React, Tailwind, and code Playground

HTML

<main>
    <section>
        <article class="first">I fill out half of the available space!</article>
        <article class="second">I fill out half of the available space! <strong>Be aware that contents of article and aside may be changed!</strong></article>
        <aside>I'm 50px width!</aside>
    </section>
</main>

CSS

main {
    background: #eee;
}

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

article {
    width: 50%;
}

.first {
    background: #cee;
}

.second {
    background: #eec;
}

aside {
    width: 50px;
    background: #ece;
    position: absolute;
    right: -25px;
}

section {
    padding: 25px;
    margin-right: 50px;
    position: relative;
}

section:after {
    content: ' ';
    display: table;
    clear: both;
}

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