JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        html, body {
            height: 100%;
            margin: 0;
            padding: 0;
        }
        .root {
            width: 60%;
            height: 100%;
            margin: auto;
            background: red;
        }
        .a {
            position: fixed;
            width: 100%;
            height: 100%;
            background: #0000ff55;
        }
    </style>
</head>
<body>
    <div class="root">
        <div class="a">
            <div class="b">
            </div>
        </div>
    </div>
</body>
</html