51)`);
    test_valid_value(`color`, `hwb(from hwb(120deg 20% 50% / .5) h w none / alpha)`,            `rgba(51, 255, 51, 0.5)`);
    test_valid_value(`color`, `hwb(from hwb(120deg 20% 50% / .5) h w b / none)`,                `rgba(51, 128, 51, 0)`);
    test_valid_value(`color`, `hwb(from hwb(120deg 20% 50% / .5) none w b / alpha)`,            `rgba(128, 51, 51, 0.5)`);
    // FIXME: Clarify with spec editors if 'none' should pass through to the constants.
    test_valid_value(`color`, `hwb(from hwb(none n