[webcomponents] Define the behavior when a custom element constructor returns an in-document element to parser (#412)

What should the HTML parser do when a custom element constructor it involved ends up inserting the element before returning to the parser?

More specifically, we need to consider the case where the returned element is a new element inserted into the document at the "right" location, a random other location, and into another document.  Should we re-adopt the element and insert into the document we're parsing?  Or should we leave the element alone?

We need to define this behavior somehow.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/412