<!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>Document</title>

    <!-- Copy this CSS below and paste it in the <head> of your document, just like in this one -->
    <style media="screen">
        html, body{
            width: 100%
        }
        .video-container{
            max-width:100%;
        }
    </style>
    <!-- end of CSS to copy -->
</head>
<body>
    <!-- Copy this code below and paste it in the <body> of your document, just like in this one -->
    <div class="video-container">
        <iframe width="560" height="315" src="https://www.youtube.com/embed/y3UH2gAhwPI" frameborder="0" allowfullscreen></iframe>
    </div>
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/fitvids/1.1.0/jquery.fitvids.js"></script>
    <script type="text/javascript">
        $(document).ready(function(){
            $('.video-container').fitVids();
        });
    </script>
    <!-- end of code to copy -->

</body>
</html>