(from lab(25 20 50 / 40%) l 0 b / alpha)`, `lab(25 0 50 / 0.4)`);
    test_valid_value(`color`, `lab(from lab(25 20 50 / 40%) l a 0 / alpha)`, `lab(25 20 0 / 0.4)`);
    test_valid_value(`color`, `lab(from lab(25 20 50 / 40%) l a b / 0)`, `lab(25 20 50 / 0)`);

    // Testing replacement with a constant.
    test_valid_value(`color`, `lab(from lab(25 20 50) 35 a b / alpha)`, `lab(35 20 50)`);
    test_valid_value(`color`, `lab(from lab(25 20 50) l 35 b / alpha)`, `lab(25 35 50)`);
    test_valid_value(`colo