ab(from oklab(0.25 0.2 0.5 / 40%) l a b / none)`, `oklab(0.25 0.2 0.5 / none)`);
    // FIXME: Clarify with spec editors if 'none' should pass through to the constants.
    test_valid_value(`color`, `oklab(from oklab(none none none) l a b)`, `oklab(0 0 0)`);
    test_valid_value(`color`, `oklab(from oklab(none none none / none) l a b / alpha)`, `oklab(0 0 0 / 0)`);
    test_valid_value(`color`, `oklab(from oklab(0.25 none 0.5) l a b)`, `oklab(0.25 0 0.5)`);
    test_valid_value(`color`, `oklab(from oklab(0.