(see the definition in the spec at http://www.w3.org/TR/css3-3d-transforms/#cssmatrix-interface)
This also means that in the Obj-C API, there's no way to make a DOMWebKitCSSMatrix at all.
getCSSMatrix() is not in the idl, and not implemented. The problem is that you can't always get a matrix if the transform value uses values that require knowledge of a renderer.
Note that zoom makes this an issue even with pixel values (e.g. see bug 24808).