I wouldn't attach game() to window, you shouldn't need to do that given what you're doing with it.
Little tip, if you are declaring a bunch of VAR in a row, you only need to say VAR once -- you can then comma delimit the list.
The delta you are using of aiming for 60fps as your average is going to have problems in browsers that don't support window.performance; when it drops through to that Date.getTime() you're going to be having that function return the same time multiple times in a row, as the Date functions timer granularity is way down at 36.4ms on most browsers. (which of course is why we have Performance in the first place). You might want to run some sort of granularity check to make sure you aren't drawing frames that don't update because of the low timer granularity (which is below 30fps). I'd suggest dropping your desired 1:1 rate (step) to 24. It runs faster, great, and it will reduce the number of systems that will run at a slower playrate thanks to falling back on getTime to nil.
That said, it's nice to see someone doing it properly, separating playrate from framerate.
Oh, and because you're going to use document and window so much, this cute trick:
Can not only save you some typing, it will actually run a hair faster since as an interpreted language, less code == faster, and shorter variable names == faster lookups.
WAIT -- if using requestAnimationFrame, the only browsers that support it also by definition support Performance -- so there's no reason to have a fallback to Date.gettime --- or is there some sort of polyfill for that we're not seeing?
I'd consider simply seeing if both requestAnimationFrame and window.performance exist, and if they don't simply bomb out with an error and lose the attempts to have fallbacks... Well, unless you REALLY care about Safari in which case you'd still need the webkitRequestAnimationFrame crap and have no window.performance; Even before Chrome forked off into Blink, the Safari builds of webkit have been such horrible code-rot lagging behind Chrome they've made IE 10+ look good on stuff like this.