<!DOCTYPE html>
<html>
<head>
    <style>
        video {
            width: 480px;
            height: 320px;
            position: absolute;
        }
        .offscreen {
            top: -320px;
        }
    </style>
    <script>
        function moveVideoOffscreen() {
            document.querySelector("video").classList.add("offscreen");
            setTimeout(function() {
                try {
                    window.webkit.messageHandlers.testHandler.postMessage("moved");
                } catch(e) {
                }
            }, 0);
        }
    </script>
</head>
<body>
    <video autoplay onplaying=moveVideoOffscreen()><source src="large-video-with-audio.mp4"></video>
</body>
<html>