In fact, the form might have multiple fields with the same validator rules but at least one of them is required. We can't use neither the notEmpty validator nor HTML 5 required attribute for all of them.
Fortunately, this can be done easily by
Using the selector option to define validators for all fields. So, we don't need to indicate the validator rules for each separate field.
Using the callback validator to check if one of fields is not empty. And then update the result of callback validator via the updateStatus() method.