requestAnimationFrame = function(callback, element) {
          var requestAnimationFrame =
            window.requestAnimationFrame        ||
            window.webkitRequestAnimationFrame  ||
            window.mozRequestAnimationFrame     ||
            window.oRequestAnimationFrame       ||
            window.msRequestAnimationFrame      ||
            function(callback) {
              return window.setTimeout(function() {
                callback(Date.now());
              }, 16);
            };
          return requestAnimationFrame.call(window, callback, element);
        }
Formats the HTML string content of the tooltip.