Requestanimationframe polyfill argument

Question

I have 2 questions regarding the requestanimationframe polyfill function:

1)Why was element included as an argument in window.requestAnimationFrame=function(callback, element)?

2)Why was 16 used here: var timeToCall =Math.max(0, 16 - (currTime - lastTime));? Is it up to the programmer judgement, not too big nor too low since we are subracting currTime-lastTime from it?

(function() {
    var lastTime =0;
    var vendors=['ms', 'moz', 'webkit', 'o'];
    for(var x=0; x<vendors.length && !window.requestAnimationFrame; ++x) {
        window.requestAnimationFrame=window[vendors[x]+'RequestAnimationFrame'];
        window.cancelAnimationFrame =
        window[vendors[x]+ 'CancelAnimationFrame'] ||
        window[vendors[x] +'CancelRequestAnimationFrame'];
    }
    if (!window.requestAnimationFrame)
    window.requestAnimationFrame=function(callback, element) {
        var currTime =new Date().getTime();
        var timeToCall =Math.max(0, 16 - (currTime - lastTime));
        var id =window.setTimeout(function() { callback(currTime+timeToCall); },
        timeToCall);
        lastTime =currTime + timeToCall;
        return id;
    };
    if (!window.cancelAnimationFrame)
    window.cancelAnimationFrame=function(id) {
        clearTimeout(id);
    };
}());

Answer 1

As stated by Hacketo, element is indeed not a parameter of the window.requestAnimationFrame, it only expects the callback function.

Secondly the 16 is used to determine a maximum FPS. The setTimeout function will not fire any faster than 60 times per second. This is however a fragile approach for game synchronization. This article describes a great approach.