gecko-dev/layout/reftests/css-ui-invalid/default-style/style.css

:not(output).invalid-ref {
  box-shadow: 0 0 1.5px 1px red;
}

:not(output):-moz-focusring.invalid-ref {
  box-shadow: 0 0 2px 2px rgba(255,0,0,0.4);
}

output.invalid-ref {
  color: red;
}

.barred-ref {
}