To be honest I'm not thrilled with the idea of doing full load like that. I was hoping for something "lighter". Sadly, I don't see a good alternative to do that in Gecko in a way fully compatible with MSHTML. I'm not sure how much we really care about this thing being exactly compatible, but if we do, I guess we need something like that...
FWIW, one alternative would be to do `document.open(); document.close();`. It would work only on HTML documents (no SVG/XML equivalent) and it replaces a document instead of creating a new one, so it's not really a good fit.
BTW, it 's hard to judge without seeing the code, but find it surprising that this thing matters for MessageEvent.source.