Re: [webcomponents] Change registries to be per document and never shared between documents (#369)

> I don't really agree. I think if you've added elements to the initial about:blank, for whatever reason, they should probably go away when you navigate.

So you get to "define" all kinds of JavaScript objects there, but not elements?

Note that the layering between DOM and HTML is already muddy. E.g., "get the parent" on documents returns the WindowProxy object. Mutation observers depend the "unit of related similar-origin browsing contexts", etc.

It's hard to make concrete, but I think in the long run we'll find that we want the registry to be per realm since there's no way to scope these constructors to documents (other than if we made it a mandatory argument, which seems rather sad). I'm not a 100% on this though, it's a hunch.

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