<object> loads say things like this:

If the URL of the given resource is not about:blank, the element's nested browsing context must then be navigated to that resource, with replacement enabled, and with the object element's node document's browsing context as the source browsing context.

In comparison, https://html.spec.whatwg.org/multipage/embedded-content.html#otherwise-steps-for-iframe-or-frame-elements says:

Navigate the element's child browsing context to url.

with no mention of what the source browsing context is. now the concept of source browsing context is broken, per #1130, but we should be indicating what the source document or whatever is here; presumably the node document of the iframe or frame element?