changed this to window in the IIFE's parameter list.

I think it's not a good idea to use "this" as the window object. 

We use Picturefill as a browserify module (Common JS syntax) which means we include it within an outer scope that is already under strict mode. 

In strict mode, "this" doesn't refer to the window object and the second argument of the IIFE (which would be the document) is undefined in our case.

It causes a JS error in the 17th line. (There is no method called createElement of undefined.)