
<!DOCTYPE html>
<html>
<head>
    <script>
        window.addEventListener('load', event => {
            let context = new webkitAudioContext();
            let node = context.createBufferSource();
            node.connect(context.destination);

            let video = document.querySelector('video');
            
            Promise.all([context.resume(), video.play()]).then(() => {
                window.webkit.messageHandlers.testHandler.postMessage('playing');
            }, () => {
                window.webkit.messageHandlers.testHandler.postMessage('not playing');
            });
        });
    </script>
</head>
    <video src="video-with-audio.mp4"></video>
</html>