       {at: 1, expect: "20px 30px 40px"},
        {at: 2, expect: "30px 40px 50px"}
      ]);

      test_interpolation({
        property: 'grid-template-columns',
        from: "10px 20px 30px",
        to: "20% 30% 40px"
      }, [
        {at: -1, expect: "calc(20px + -20%) calc(40px + -30%) 20px"},
        {at: 0, expect: "calc(10px + 0%) calc(20px + 0%) 30px"},
        {at: 0.4, expect: "calc(6px + 8%) calc(12px + 12%) 34px"},
        {at: 0.6, expect: "calc(4px + 12%) calc(8px + 18%) 36px"},
        