[css-color-5] explicitely test that using rgb channel keywords in xyz is invalid (#43877)

* [css-color-5] explicitly that using rgb channel keywords in xyz is invalid

* Update color-invalid-relative-color.html
diff --git a/css/css-color/parsing/color-invalid-relative-color.html b/css/css-color/parsing/color-invalid-relative-color.html
index 9746d6e..3cab1ff 100644
--- a/css/css-color/parsing/color-invalid-relative-color.html
+++ b/css/css-color/parsing/color-invalid-relative-color.html
@@ -100,6 +100,7 @@
         test_invalid_value(`color`, `color(from color(${colorSpace} 0.7 0.5 0.3) ${colorSpace} red g b)`);
         test_invalid_value(`color`, `color(from color(${colorSpace} 0.7 0.5 0.3) ${colorSpace} x g b)`);
         test_invalid_value(`color`, `color(from color(${colorSpace} 0.7 0.5 0.3) ${colorSpace} l g b)`);
+        test_invalid_value(`color`, `color(from color(${colorSpace} 0.7 0.5 0.3) ${colorSpace} x y z)`);
     }
 
     for (const colorSpace of [ "xyz", "xyz-d50", "xyz-d65" ]) {
@@ -113,6 +114,7 @@
         test_invalid_value(`color`, `color(from color(${colorSpace} 7 -20.5 100) ${colorSpace} red y)`);
         test_invalid_value(`color`, `color(from color(${colorSpace} 7 -20.5 100) ${colorSpace} r y z)`);
         test_invalid_value(`color`, `color(from color(${colorSpace} 7 -20.5 100) ${colorSpace} l y z)`);
+        test_invalid_value(`color`, `color(from color(${colorSpace} 7 -20.5 100) ${colorSpace} r g b)`);
     }
 </script>
 </body>