purple r g b / none)`,                           `rgba(102, 51, 153, 0)`);
    test_valid_value(`color`, `rgb(from rgb(20% 40% 60% / 80%) r g none / alpha)`,              `rgba(51, 102, 0, 0.8)`);
    test_valid_value(`color`, `rgb(from rgb(20% 40% 60% / 80%) r g b / none)`,                  `rgba(51, 102, 153, 0)`);
    // FIXME: Clarify with spec editors if 'none' should pass through to the constants.
    test_valid_value(`color`, `rgb(from rgb(none none none) r g b)`,                            `rgb(0, 0