 implements IronCheckedElementBehaviour,
        // in which case like the native checkbox/radio button, it's only used
        // when checked.
        // For native elements, from https://www.w3.org/TR/html5/forms.html#the-form-element.
        // Native submittable elements: button, input, keygen, object, select, textarea;
        // 1. We will skip `keygen and `object` for this iteration, and deal with
        // them if they're actually required.
        // 2. <button> and <textarea> have a `value` pro