<script>
if (window.layoutTestController)
    layoutTestController.dumpAsText();
</script>
This test passes if the activeElement is the body rather than the input element
(which it would be if the sandbox didn't succeed in blocking autofocus).
<iframe sandbox="allow-scripts"
    src="data:text/html,<input autofocus onfocus><script>alert(document.activeElement.tagName)</script>"></iframe>