<html>
    <head>
        <script>
            function pageLoaded() {
                try {
                    window.webkit.messageHandlers.testHandler.postMessage("loaded");
                } catch(e) { }
            }

            function beganPlaying() {
                try {
                    window.webkit.messageHandlers.testHandler.postMessage("played");
                } catch(e) { }
            }

            function play() {
                let audio = document.getElementById("audio");
                audio.playbackRate = 5; // Speeds up the test.
                audio.play();
            }
        </script>
    </head>
    <body onload="pageLoaded()">
        <button onclick="play()">Play</button>
        <br />
        <audio controls id="audio" onplaying="beganPlaying()" src="silence-long.m4a" />
    </body>
</html>