.mktoErrorMsg {
  display: none;
  width: 100%;
  margin-top: 0.25rem;
  font-size: 0.875em;
  color: #dc3545;
}

.was-validated :invalid ~ .mktoErrorMsg {
  display: block;
}

.form-check-inline .form-check-input ~ .mktoErrorMsg {
  margin-left: .5em;
}