CustomStateSet: add() method

The add method of the CustomStateSet interface adds value representing a custom state to the CustomStateSet.

Custom elements with a specific state can be selected using the :state() pseudo-class, specifying the desired state as an argument.

add(value)

class MyCustomElement extends HTMLElement {
  set checked(flag) {
    if (flag) {
      this._internals.states.add("checked");
    }
  }
}