chain.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="chain.js"></script>
    <style rel="stylesheet">
        * { margin: 0; padding: 0;}
        html, body {background-color: #3a3a3a; color: #5f5f5f}
        .container {
            display: flex;
            max-width: 100%;
            max-height: 300px;
            flex-wrap: wrap;
        }
        .box {
            width: 20px;
            height: 20px;
            background: #2b2b2b;
            padding: 10px;
            text-align: center;
            color: #e0e0e0;
            font-size: 12px;
            border: 2px solid #3a3a3a;
            outline: 3px solid #1d1d1d;
        }
        .box:hover {
            background: #282828;
            border: 2px solid #1d1d1d;
            outline: 3px solid #3a3a3a;
        }
    </style>
</head>
<body>

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

</body>
</html>