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It feels very tedious and complicated to have shadow parents etc for custom elements extending other custom elements. I think it introduces complications that might not be needed for a first draft of the spec?
I agree! It would've been much simpler to not have multiple shadow DOM trees. In fact, I tried to resist having multiple trees for the longest time. Unfortunately, without multiple shadows, it is impossible to clearly explain how to add shadow DOM subtrees to a bunch of built-in elements that already have shadow DOM-ish behavior. And so there we are :-\