index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Animation Test</title>
    <style>
        @keyframes x{
            100%{width: 500px}
        }
        @keyframes y {
            100%{height: 200px}
        }
        .b{
            width: 100px;
            height: 100px;
            background-color: lightblue;
            margin: 20px;  
        }
    </style>
</head>
<body>
    <input type="submit" id="btn" value="fadeTo">
    <input type="submit" id="btn2" value="transform">
    <div id="a" class="b"></div>
    <div id="c" class="b"></div>
    <script src="animation.js"></script>
    <script>
        let btn = document.getElementById('btn'),
            div = $(document.getElementById('a')),
            btn2 = document.getElementById('btn2'),
            div2 = $(document.getElementById('c'))
        btn.onclick = function() {
            div2.fadeTo(0.2)
        }
        btn2.onclick = function() {
            div.animate('width: 200px')
                .animate('height: 200px')
                .animate('width: 100px')
                .animate('height: 100px')
        }
    </script>
</body>
</html>