JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrap">
    <div class="a"></div>
    <div class="b"></div>
</div>

CSS

.wrap, .wrap * {
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    box-sizing: border-box;
}

.wrap {
    width: 300px;
    border: 2px solid #000;
    overflow: hidden;
}

.wrap .a {
    width: 20%;
    background: #f00;
    height: 100px;
    float: left;
    border: 5px solid #00f;
}

.wrap .b {
    width: 80%;
    background: #0f0;
    height: 100px;
    float: left;
    border: 5px solid #00f;
}