Re: [w3c/webcomponents] Should custom elements be adoptable if so, how? (#512)

> Do you see a path toward us getting more information in the future, which we don't have now, that allows us to converge on another option? If we have all the information now, punting by choosing throw seems wrong.

I don't think it's a matter of information as much as a matter of consensus.  We can't reach a consensus on custom elements behavior before reaching a consensus on builtin elements.

> the adoption agency algorithm

We investigated the adoption agency algorithm in the past and found that it can't create a custom element although it's possible to have `is=` on them.  This was yet another reason we didn't want to support `is=` since it's just not possible to run arbitrary scripts during the adoption agency algorithm safely.

> I guess in general we have to worry about the fact that if adopt can throw, any insertion-based operation (insert, append, replace) can also throw now.

This is generally true already because [**pre-insert**](https://dom.spec.whatwg.org/#concept-node-pre-insert) needs to [**ensure pre-insertion validity**](https://dom.spec.whatwg.org/#concept-node-ensure-pre-insertion-validity) which can throw a bunch of exceptions such as `HierarchyRequestError` and `NotFoundError`.

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