Today I came across a CSS 3D Lighting library which uses neither of these. Of course the basic necessity of 3D rendering is the Z-axis, but lighting is equally important. Without appropriate lighting and/or materials you end up with a silhouette that most likely makes no sense. The lighting can enhance your z-depth, and the eye will react more naturally. Then we can begin to apply blurs to really finish out the depth-of-field.

Photon: CSS 3D Lighting
http://photon.attasi.com/