Are 2D transforms hardware accelerated in Mobile S

I've often been told that CSS 3D transforms are hardware accelerated in Mobile Safari which makes me wonder if the implication is that 2D transforms are not? I can think of no reason why they wouldn't be, since they can basically all be implemented as 3D transforms, but I would like to know for sure.

If it turns out that 2D transforms are not hardware accelerated, any insight as to why would be much appreciated.

You're right, CSS 2D transforms aren't hardware accelerated in Mobile Safari, but 3D transforms are. I'm not sure why it's that way, but perhaps they decided it was overkill for most 2D transforms. Using the GPU unnecessarily could adversely affect battery life.

It's very easy to convert a 2D transform to a 3D transform so it isn't much of a problem. One trick is to use translateZ(0) as described here: http://creativejs.com/2011/12/day-2-gpu-accelerate-your-dom-elements/

EDIT

Apple doesn't say anything about it in their documentation, so it is difficult to get an authoritative source. Here is what Dean Jackson from Apple had to say about it (from http://mir.aculo.us/2010/08/05/html5-buzzwords-in-action/):

In essence, any transform that has a 3D operation as one of its functions will trigger hardware compositing, even when the actual transform is 2D, or not doing anything at all (such as translate3d(0,0,0)). Note this is just current behaviour, and could change in the future (which is why we don’t document or encourage it). But it is very helpful in some situations and can significantly improve redraw performance.

Ariya Hidayat from Sencha wrote a post explaining hardware acceleration on mobile browsers: http://www.sencha.com/blog/understanding-hardware-acceleration-on-mobile-browsers/. Here's a snippet from the post:

The best practice of setting the CSS transformation matrix to translate3d or scale3d (even though there is no 3-D involved) comes from the fact that those types of matrix will switch the animated element to have its own layer which will then be composited together with the rest of the web page and other layers. But you should note that creating and compositing layers come with a price, namely memory allocation. It is not wise to blindly composite every little element in the web page for the sake of hardware acceleration, you’ll eat memory.

Here is an article from html5rocks.com that discusses hardware acceleration: http://www.html5rocks.com/en/tutorials/speed/html5/. Here's a snippet from it:

Currently most browsers only use GPU acceleration when they have a strong indication that an HTML element would benefit from it. The strongest indication is that a 3D transformation was applied to it. Now you might not really want to apply a 3D transformation, but still gain the benefits from GPU acceleration - no problem. Simply apply the identity transformation:

-webkit-transform: translateZ(0);

Firefox and Internet Explorer already use hardware acceleration for 2D transforms, so I wouldn't be surprised if the WebKit browsers (Chrome, Safari) include it in the near future.