code: function () {
                      if (parserInput.peekNotNumeric()) {
                          return;
                      }
  
                      var value = parserInput.$re(/^([+-]?\d*\.?\d+)(%|[a-z_]+)?/i);
                      if (value) {
                          return new(tree.Dimension)(value[1], value[2]);
                      }
                  }
docs: A Dimension, that is, a number and a unit  0.5em 95%