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<html>
<body>
<script>
var onloadFiredAt;
let start = performance.now();
while (performance.now() - start < 5000);
window.onload = function() {
onloadFiredAt = performance.now();
}
window.addEventListener('message', function(event) {
window.parent.postMessage(onloadFiredAt, "*");
})
</script>
</body>
</html>