Add platform-provided mixins explainer

[anaskim]

V1 includes:

• Rewording on interoperability.
• Adds examples on current workarounds used by frameworks.
• Adds goal to clarify the general and specific objectives of the proposal.
• "behavior" -> "mixin", to avoid confusion with "behaviour" spelling.
• Simplifies to add mixins to attachInternals() and avoid dynamic changes at runtime.
• Adds mixins property to ElementInternals to query behaviors.
• Removed "Open questions" section and expanded "Future Work" section with a "collision handling" note and 2 more proposed mixins (input and table behaviors).
• Updated "Accessibility, Security, and Privacy Considerations" section.
• Updated acknowledgements.

[KurtCattiSchmidt]

Another pro to this approach is that it allows for child classes to easily extend from mixins. e.g.

class CustomSubmitButton extends HTMLSubmitButtonMixin(HTMLElement) { ... }
class ExtraCustomSubmitButton extends CustomSubmitButton {...}

Is this possible with the current mixin proposal?

[anaskim]

Yes, we talked about in the meeting about child classes inheriting ElementInternals. I tested it and child classes do inherit the instance and can modify it. With the proposed approach we could have:

class ParentElement extends HTMLElement {
  #internals;

  constructor() {
    super();
    this.#internals = this.attachInternals();
    // ...
    this._internals = this.attachInternals({ mixins });
  }

  _getInternals() {
    return this.#internals;
  }
}

class ChildElement extends ParentElement {
  constructor() {
    super();
    const internals = this._getInternals();
    internals.role = 'checkbox'; 
    const isSubmit = this._internals.mixins.includes(HTMLSubmitButtonMixin);
    if (isSubmit) {
      // ...
    }
  }
}

[KurtCattiSchmidt]

Does it make sense to list "Extend Markup Like Customizable Select"? Or is that too different of a problem? The dealbreaking con with this approach is that it won't work for all types (or any types beyond select?).

https://developer.mozilla.org/en-US/docs/Learn_web_development/Extensions/Forms/Customizable_select

[anaskim]

Yes, added it as an alternative. PTAL.