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If an unresolved element had been garbage-collected, it probably doesn't need its lifecycle callbacks called. The spec should say something about that.
The key here is to neither guarantee that the callbacks will be called, or that they won't be called.
I decided to steal the language from http://dom.spec.whatwg.org/#garbage-collection https://dvcs.w3.org/hg/webcomponents/rev/7e22811000df