Re: [webcomponents] Change registries to be per document and never shared between documents (#369)

@annevk : It would mean that custom element's constructor would see a wrong `ownerDocument` until it gets adopted into that document.

It's not a huge deal per se but it's yet another edge case author has to know / be aware of.  We think it's better to disallow it altogether in the first place so that the behavior is consistent and easy to understand.

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/369#issuecomment-192112878