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

Upon sleeping on this I am amenable to a new alternative, which is a _mandatory_ document parameter to all custom element constructors. So:

- HTMLElement takes a mandatory first argument which is the document in which it is created.
- super() calls inside custom element constructors must pass that first argument. This is probably easiest accomplished by doing `constructor(...args) { super(...args); ... }`, or maybe `constructor(document, optionalSpecialStuff) { super(document); ... }`
- The parser and createElement pass the relevant document as the first parameter to the custom element constructor (`new CustomElement(relevantDocument)`)
- As a corollary, in general `new CustomElement()` will not work.
  - This should make the `new CustomElement(document)` <-> `document.createElement("custom-element")` connection more obvious. They are simply alternative syntaxes for each other.
  - `new CustomElement()` could be made to work by authors if they do something like `constructor() { super(window.document); }`.
  - But doing this is opting in to a behavior which prevents your element from being used correctly when registered on other documents.
- This is forward-compatible with future expansions like passing an optional tag name (which I would still prefer to be in a dictionary).

I am not OK with it defaulting to some specific document because it is ambiguous which document. We either:

1. Should make this safe for authors by ensuring a 1:1 window:custom element registration setup (e.g. by sharing or disallowing registries in non-windowed documents), so that `new CustomElement()` is unambiguous
2. Or should make this an explicit mandatory part of the API that authors need to be aware of.

WDYT? (1) or (2)?

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/369#issuecomment-191754665