<!DOCTYPE html>
<body>
<script>
window.addEventListener("message", function (event) {
    window.webkit.messageHandlers.testSystemPreview.postMessage({ message: "triggered", action: event.data });
});

window.addEventListener("load", function () {
    let pageID = internals.pageIdentifier(document);
    let frameID = internals.frameIdentifier(document);
    let msg = { message: "loaded", pageID, frameID };
    window.webkit.messageHandlers.testSystemPreview.postMessage(msg);
});

</script>