Should children of an element with -webkit-transform-style set to flat draw in document order or be depth sorted? Safari depth sorts whereas Chrome's compositor doesn't. My take is that unless the transform-style of the parent is set to preserves-3d, children should be drawn in their document (or z-index) order but I don't think the spec is clear about this. This page demonstrates the difference: http://dl.dropbox.com/u/2176065/perspective/index.html#album-4 (you'll need to mouse over the covers on the right to get them to flip around)
I think that the elements have to be painted in document order (or more precisely the usual CSS painting order) unless CSS specifies exactly what Safari's depth sort does, which seems somewhat impractical.
-webkit-transform-style: flat should draw in document order, with no intersection. Safari isn't able to do this correctly yet.
Just wanted to report that this is still an issue, and is causing us considerable headaches when building a WebGL AR/VR application which needs to transform the HUD, which is partially written with normal DOM elements.