UXP/layout/reftests/css-valid/select/style.css

/* Override default style */
select:invalid {
  box-shadow: none;
}

select.notvalid {
  background-color: green;
}
select.notvalid:valid {
  background-color: red;
}

select.valid {
  background-color: red;
}
select.valid:valid {
  background-color: green;
}