[Konstantin Roman (nihil-pro)]

fix(selectors): support labels for any correctly form-associated custom elements

"labels" property exists only for INPUT, BUTTON, METER, OUTPUT, PROGRESS, SELECT and TEXTAREA; but hardcoding these names limits custom elements.

Any custom element that wants to be a real form element must also implement a "labels" getter:

class CustomFormElement extends HTMLElement {
  static formAssociated = true;
  #internals;

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

  get labels() {
    return this.#internals.labels;
  }
}

Also, for a hidden input, the "labels" getter will return null. No reasons to double check.
So this function can be simplified dramatically: return Array.from(Reflect.get(element, "labels") || []);

Fixes #43898

[Konstantin Roman (nihil-pro)]

@microsoft-github-policy-service agree