To be clear, this FAQ isn't actually part of the spec (which I'm counting as the document here: https://wicg.github.io/portals/).
https://github.com/WICG/portals/blob/master/explainer.md#ifr...
Heh, "we considered this but it's hard to do in Chrome".
But they don't give a shit if <portal> is hard to do in Gecko, of course.
https://github.com/WICG/portals/blob/master/explainer.md#faq...
https://github.com/WICG/portals/blob/master/explainer.md#alt...