<object id="obj1" data="blue.html" name="o" height="50" width="100"></object> should expose a contentWindow. See https://html.spec.whatwg.org/multipage/embedded-content.html#dom-object-contentwindow This bug was found by the newly added test: LayoutTests/http/tests/w3c/html/semantics/embedded-content/the-object-element/object-attributes.html
<rdar://problem/22590264>
I think this works now, based on: https://wpt.fyi/results/html/semantics/embedded-content/the-object-element/object-attributes.html?label=experimental&label=master&aligned