* emToPx, epsilon, "horizontal characters don't stretch vertically");

          element = document.getElementById("non_stretchy_vertical_arrow");
          assert_approx_equals(element.getBoundingClientRect().height, 1 * emToPx, epsilon, "vertical characters stretch vertically");
      }, `Taking into account stretchy property.`);

      test(function() {
          // There are only stretchy operators : the maximum of their base sizes
          // is used as the target size.
          let tallest_base_size