LayoutTests/fast/events/beforeload-assertion.html

<div id="d1"></div>
<script>
if (window.testRunner)
    testRunner.dumpAsText();
document.addEventListener('beforeload', function(event) {
    d1.appendChild(document.createElement('button'));
    d1.offsetLeft;
    d1.remove();
}, true);
</script>
<embed src="data:image/png,"></embed>
<p>PASS if no assertion failure.</p>