Untitled

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        .null{
            transition:1s;
            opacity: 0;
            width: 30px;
            height: 30px;
            background: white;
            display: inline-block;
            margin: 10px;
        }
        .box{
            width: 30px;
            height: 30px;
            background: green;
            display: inline-block;
            margin: 10px;
            opacity: 1;
        }
    </style>
</head>
<body>
    <script>
        let el = document.createElement('div')
        document.body.appendChild(el)
        el.classList.add('null')
        setTimeout(function(){el.classList.add('box')}, 10)
    </script>
</body>
</html>