  test_valid_value(`color`, `lab(from lab(25 20 50 / 40%) l a b / none)`, `lab(25 20 50 / none)`);
    // FIXME: Clarify with spec editors if 'none' should pass through to the constants.
    test_valid_value(`color`, `lab(from lab(none none none) l a b)`, `lab(0 0 0)`);
    test_valid_value(`color`, `lab(from lab(none none none / none) l a b / alpha)`, `lab(0 0 0 / 0)`);
    test_valid_value(`color`, `lab(from lab(25 none 50) l a b)`, `lab(25 0 50)`);
    test_valid_value(`color`, `lab(from lab(25 20 50 / n