Re: [webcomponents] Define the behavior for sync element creation when a custom element constructor does various bad things (#412)

My initial thought is that we should check all of these, because they seem like good things to enforce. But, would that be a performance worry?

Maybe it's better to just tell people to not do dumb things, and if they do, then their documents get weird? I don't think we're protecting any important invariants here, are we?

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