<script>
if (window.testRunner) {
    testRunner.dumpAsText();
    testRunner.waitUntilDone();
}
</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"
    src="data:text/html,<input autofocus onfocus='console.log(document.activeElement.tagName); testRunner.notifyDone();'>"></iframe>