JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <div id="background"></div>
    <header>
        <ul>
            <li>Button 1</li>
            <li>Button 2</li>
        </ul>
    </header>
</body>

CSS

* {
    margin:0;
    padding:0;
}
html {
    width:100%;
    height:100%;
}
body {
    width:100%;
    height:100%;
}
#background {
    width:100%;
    height:200px;
    background:#989898;
    z-index:-1;
}
header {
    width:100%;
    height:100px;
    margin-top:50px;
    background:rgba(0, 0, 0, 0.4);
}