index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Dropdown Menu - animation effect</title>
    <link rel="stylesheet" href="style.css">
</head>
<body>
    <div class="container">

        <div class="dropdown-menu">
            <button class="dropdown-toggle">Topics</button>

            <ul class="dropdown-list">
                <li class="dropdown-list__item" style="--delay:0.2s">Front-end</li>
                <li class="dropdown-list__item" style="--delay:0.4s">Back-end</li>
                <li class="dropdown-list__item" style="--delay:0.6s">SEO</li>
                <li class="dropdown-list__item" style="--delay:0.8s">Digital marketing</li>
            </ul>
        </div>

    </div>
</body>
</html>