async_test(t => { const frame = document.body.appendChild(document.createElement("iframe")); t.add_cleanup(() => { frame.remove(); }); const originalHTMLElement = frame.contentDocument.documentElement; assert_equals(originalHTMLElement.localName, "html"); const observer = new frame.contentWindow.MutationObserver(t.step_func_done(records => { // Even though we passed `subtree: true` to observer.observe, due to the // fact that "replace all" algorithm removes children with the "suppress // observers flag" set, we still only get the html element as the sole // removed node. assert_equals(records.length, 1); assert_equals(records[0].type, "childList"); assert_equals(records[0].target, frame.contentDocument); assert_array_equals(records[0].addedNodes, []); assert_array_equals(records[0].removedNodes, [originalHTMLElement]); })); observer.observe(frame.contentDocument, { childList: true, subtree: true }); assert_equals(frame.contentDocument.open(), frame.contentDocument); }, "document.open() should inform mutation observer of node removal");