JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>TOP</title>
    <link rel="stylesheet" href="style.css">
</head>
<body>
    <div class="but">
        <button>BUTTON</button>
    </div>
    <script type="text/javascript" src="ler.js"></script>
</body>
</html>

CSS

div{
    width: 100%;
    height: 100%;
    position: fixed;
    top: 0;
    left: 0;
    display: flex;
    align-items: center;
    align-content: center; 
    justify-content: center; 
    overflow: auto;   
}
button {
    display: block;
    background: blue;
}