LayoutTests/http/tests/media/media-stream/resources/device-change-iframe.html

<script>
    let myID;
    navigator.mediaDevices.ondevicechange = (event) => {
        parent.postMessage(myID, '*');
        result.innerHTML = `${myID} got 'devicechange' event`;
    };
    window.addEventListener("message", (event) => { myID = event.data; });
</script>

<div id='result'></div>