Re: [w3c/webcomponents] Add authoring guidance for writing custom element constructors (#409)

> would need to use a private flag / conditional in the connectedCallback

Yes, they would. This should be something done very rarely anyway, so making it inconvenient to do so is not a real concern.

> Would setting innerHTML or calling appendChild in a constructor throw an error? 

Yes, or more precisely, it would cause element creation to fail, so `createElement` or the parser or whatever would fail to create the element.

> The one given is that it "violates expectations" but I'm unclear on what those expectations are - can anyone elaborate?

So far in the history of the DOM, every element ever returned from `createElement` has had no attributes and no children. The expectation is that this invariant can be relied on going forward.

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