Should z-index apply to transformed elements?

In the 2D transforms spec <http://dev.w3.org/csswg/css3-2d-transforms/> we say that applying the transform property to an element makes it behave like a position: relative element in the sense that it acts as a positioning container.

The question is whether it should also behave like position: relative in the sense that z-index applies. WebKit does not implement this, but Boris suggests that we do <https://bugs.webkit.org/show_bug.cgi?id=57730>.

Simon