index.html

<!DOCTYPE html>
<html>
    <head>
        <title>Äpfel und Birnen</title>
        <link rel="stylesheet" href="css/style.css" type="text/css" />
    </head>
    <body>
        <div id="content">
            <a href="#" id="fs">Vollbild</a>
        </div>

        <script type="text/javascript">

        function fs_status() {
            if(document.fullscreenElement) {
                return true;
            } else if(document.webkitFullscreenElement) {
                return true;
            } else if(document.mozFullScreenElement) {
                return true;
            } else {
                return false;
            }
        }

        function goFullscreen() {
            if(fs_status()) { return; }

            var el = document.getElementsByTagName('canvas')[0];
            var requestFullScreen = el.requestFullscreen || el.msRequestFullscreen || el.mozRequestFullScreen || el.webkitRequestFullscreen;

            if(requestFullScreen) {
                requestFullScreen.call(el);
            }

        }

        document.getElementById('fs').addEventListener('click', goFullscreen);
	   </script>
    </body>
</html>