     test_interpolation({
        property: 'grid-template-columns',
        from: "inherit",
        to: "20px 30px 40px"
      }, [
        {at: -1, expect: "0px 10px 20px"},
        {at: 0, expect: "10px 20px 30px"},
        {at: 0.4, expect: "14px 24px 34px"},
        {at: 0.6, expect: "16px 26px 36px"},
        {at: 1, expect: "20px 30px 40px"},
        {at: 2, expect: "30px 40px 50px"}
      ]);

      // Exercise <track-list> (with <track-repeat>)
      // https://drafts.csswg.org/css-grid/#repeat-in