<script>
if (window.testRunner)
    testRunner.dumpAsText();
</script>
This test passes if the activeElement is the input element rather than the body
(which it would be if the sandbox didn't allow autofocus although allow-scripts flag is set).
<iframe sandbox="allow-scripts allow-modals"
    src="data:text/html,<input autofocus onfocus><script>window.onload = function() { alert(document.activeElement.tagName) }</script>"></iframe>