Why did Guil use rotateZ and not a normal rotate? For the GPU acceleration boost?

There is no difference in what they do, they both rotate on the z axis. But I believe rotateZ does trigger hardware acceleration in webkit browsers as it utilizes the GPU instead.

For example you can actually trigger hardware acceleration on 2D transforms by including a 3D transform even if the value is set to 0.

#example{
   transform: rotate(90deg) translateZ(0);
}

The above code would trigger hardware acceleration due to translateZ();

This allows you to have a 2D rotate that is backwards compatible with browsers that do not support 3D CSS transforms. and also at the same time enable hardware acceleration in browsers that do. translateZ(0); will not break the rotate in browsers that are not compatible with 3D Transforms.

You can do this with any of the 3D CSS transforms:

    translate3d()
    translateZ()
    scale3d()
    scaleZ()
    rotateX()
    rotateY()
    rotate3d()
    perspective()
    matrix3d()