Vendor prefixes are a funny thing. Mix them with global WebIDL own properties and it gets weird.

From blink-dev,

var requestAnimationFrame = window.webkitRequestAnimationFrame ||
    window.opRequestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    window.msRequestAnimationFrame ||
    window.requestAnimationFrame;

If you run that in Chrome it probably won’t do what you think it might.

> requestAnimationFrame
function webkitRequestAnimationFrame() { [native code] }

So don’t hold your breath waiting for webkitRequestAnimationFrame() to get removed any time soon.

Now from bugzilla,

var requestAnimationFrame = function (win, t) {
      return win["webkitR" + t] || win["r" + t] || win["mozR" + t]
          || win["msR" + t] || function (fn) {
          setTimeout(fn, 1000 / 60)
      }
  }(window, "equestAnimationFrame");

Let me quote bz,

Before bug 932322, this used to shadow the default requestAnimationFrame with undefined and therefore pick up the mozRequestAnimationFrame. Now it picks up the default requestAnimationFrame.

In Chrome, it picks up webkitRequestAnimationFrame, since the code puts that before the standard version, unlike mozRequestAnimationFrame.

If I comment out this code so that the standard requestAnimationFrame is used in all browsers, the page stops working in Chrome as well. If instead I change that code to put the moz-prefixed version next to the webkit-prefixed one, so it’s picked up in preference to the standard one, then the page works in Firefox.

And now I’ll quote Erik Arvidsson from the blink-dev thread as to the why for both (ignoring the DOMHighResTimeStamp problem in the bugzilla issue),

Remember that VariableDeclarations hoist and are initialized to undefined, as well as all global bindings become own properties on the global object (which is the window in a web browser).

There are a few solutions (Adam mentioned one).

1. Do not use global variables. The original code is fine inside a function scope.
2. Use window.requestAnimationFrame = ...
3. You can also skip the var if you are in non-strict mode.

Fun times on the internet.