Re: [whatwg/dom] shadowRoot.innerHTML parsing elements from another realm/iframe (#977)

> I think the `<p>` case is still not clear though, since step 7.1 of [create an element](https://dom.spec.whatwg.org/#concept-create-element) is not specific about which window to look up the element interface in. (I guess the HTMLElement and HTMLUnknownElement in step 6 has a similar problem, for the intermediate period between the insertion and the upgrade...). I'd say there are at least two plausible choices:

Yeah, none of those specify in which realm we'd be looking up the interfaces, which is ambiguous given what you've discovered.

> * Use the element interface from the global associated to the document passed to "create an element"
> * Use the element interface from the current global object (i.e. the global object of the innerHTML setter)
> 
> In this case they are equivalent and equal to the other window (so `<p>` should not be from the main window, I think, so Chrome's behavior is correct?).

I'm not following. Both of those options seems to result in creating `HTMLParagraphElement` in the realm of the main window since the shadow host is in the main document at that point? The only reasonable option I can think of creating `HTMLParagraphElement` of the iframe's realm will be if we did the interface lookup via the global object of `this` in `innerHTML`'s setter.

Arguably this behavior is a bit strange / unlike other JS objects since the following code will return `false` but I think it's a necessary oddity that arises from the fact `Node` has the ability to get adopted to another document and change its associated document:
```
iframeRange = document.body.appendChild(document.createElement('iframe')).contentDocument.createRange();
iframeRange.selectNode(document.body);
iframeRange.cloneRange().__proto__ == document.createRange().__proto__
```

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