The following issues need APIs which should be used by custom element authors and should not be used by custom element users.

• Need callback for form submit data (#187)
• Custom pseudo-classes for host elements via shadow roots (:state) (#738)

At the March F2F we came up with one idea. However using ShadowRoot for such APIs looks very weird to me and I'd like to discuss it again.

Candidates:

• ShadowRoot
  Pros: Usually only custom element implementation knows the instance of ShadowRoot which is used to implement the custom element. It's difficult for custom element users to get a ShadowRoot instance created by a custom element implementation [1]
  Cons: ShadowRoot is an interface for tree-encapsulation. Adding features unrelated to tree-encapsulation looks like a design defect. We should not make ShadowRoot a kitchen sink.
  Cons: Not all custom element implementations need ShadowRoot. For example, a checkbox custom element won't need ShadowRoot. Creating unnecessary ShadowRoot for such APIs is not reasonable.
  Cons: If a custom element implementation uses no ShadowRoot, a custom element user can call element.attachShadow() to get a ShadowRoot instance, and thus get access to these private APIs.

• new Something(customElement)
  It throws if users try to create it for the same element twice. It throws if the constructor is called with non-custom elements. The interface Something is called as ElementStates in Custom pseudo-classes for host elements via shadow roots (:state) #738, and called as HTMLElementPrimitives in Need callback for form submit data #187.
  Pros: ShadowRoot won't have unrelated APIs.
  Pros: Usually only custom element implementation knows the instance of Something. It's difficult for custom element users to get a Something instance created by a custom element implementation [1]
  Cons: If a custom element implementation uses no Something, a custom element user can call new Something(element) successfully to get a Something instance.

• Deliver a Something instance by a custom element callback
  See Need callback for form submit data #187 (comment) for more details.
  Pros: ShadowRoot won't have unrelated APIs.
  Pros: Custom element users can not create a Something instance unlike other two candidates.
  Pros: It's difficult for custom element users to get a Something instance delivered to a custom element [1]
  Cons: UA implementation would need larger code for a new callback, compared to the other two candidates.

IMO, the second one or the third one is much better than the first one.

@annevk @domenic @rniwa @trusktr What do you think?

[1] If a custom element implementation stores a ShadowRoot / Something instance to this.foo_, a custom element user can get the instance by accessing yourElement.foo_. There are some techniques to avoid such casual access.