Choppy scrolling in Safari

I haven't checked to see how my answer compares to Jack's, but I think the problem is that Safari attempts to be very power efficient. As a result, it is hesitant to enable hardware acceleration unless it needs to. A common trick that people use to force hardware acceleration is to place -webkit-transform: translate3d(0, 0, 0); into the css for the divs which are moving. I tried it with the content class and it seemed a little better. You might try applying it to the other layers as well.

EDIT: I applied it to the left and right text holder divs as well, and the page seems just as smooth as Chrome now.

I took a look and did see the "choppy" scrolling you mentioned (looking at it more, it was hit or miss - sometimes it was smooth, other times it was VERY choppy).

It seems you've got some performance issues with your parallax callback on Safari (though it wouldn't surprise me if it's some buggy implementation with Safari...)

One thing I would recommend is taking a look at requestAnimationFrame for webkit. For a test, I wrapped the logic to update the offsets in a raf (and cached the window.pageYOffset value) and it seemed smoother on my end.

function parallax(e) {
    window.webkitRequestAnimationFrame(function() {
        var offset = window.pageYOffset;
        a.style.top = (offset / 2) + "px";
        b.style.top = (offset / 2) + "px";
        textbox.style.top =- (offset * 0.7) + "px";
        textbox2.style.top =- (offset * 0.7) + "px";
    });
}

To be honest, you could probably use raf for all browsers (if they support it).

Another trick people use when animating elements is to accelerate the layer that the element you are animating is on. There are a few ways to do this, but the easiest is to use -webkit-transition and set translateZ(0). It wouldn't hurt to add the two additional lines as well:

-webkit-backface-visibility: hidden;
-webkit-perspective: 1000;

Also, I noticed that some elements you offset (using style) are position: relative - Personally, I'd say that any element that's to be animated should be position: absolute. This will remove the element from the DOM, and offsetting it won't cause reflows to surrounding elements (which may contribute to your choppiness).

Edit - one other thing I noticed is that "choppiness/weirdness" happens when you encounter rubberbanding on safari (my guess are the negative values). That might be something you'll want to look at as well.

Good luck!