JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <div class="container">
        <div class="sidebar">
        </div>
    </div>
</body>

CSS

*, *:before, *:after {
    box-sizing: border-box;
}

html, body, div {
    background: rgba(0, 0, 0, .2);
    margin: 0;
    padding: 20px;
    height: 100%;
    border: 1px solid #000;
}

.container {
    height: 250px;
}
.sidebar {
    width: 20%;
}