code: def click(self, selector, btn=0):
          
          return self.evaluate("""
              (function () {{
                  var element = document.querySelector({0});
                  var evt = document.createEvent("MouseEvents");
                  evt.initMouseEvent("click", true, true, window, 1, 1, 1, 1, 1,
                      false, false, false, false, {1}, element);
                  return element.dispatchEvent(evt);
              }})();
          """.format(repr(selector), repr(btn)))
docs: Click the targeted element.
  
          :param selector: A CSS3 selector to targeted element.
          :param btn: The number of mouse button.
              0 - left button,
              1 - middle button,
              2 - right button