[css3-3d-transforms] Spec questions

While implementing the 3d transforms spec for Mozilla I've come up with 
a few questions/suggestions:

- How does preserve-3d affect the definition of transform's containing 
block/stacking context?

    When we have a multiple level hierarchy of preserve-3d elements,
    should depth sorting stay constrained to to each stacking context,
    or should all elements be moved up into the root-most stacking
    context before sorting.

    Using the WebKit 'poster-circle'
    (http://www.webkit.org/blog-files/3d-transforms/poster-circle.html)
    as an example, retaining stacking contexts will sort the 12 posters
    by depth position within a ring, and the 3 rings (as a whole) within
    the cylinder. This results in 'broken' rendering since the rings can
    still end up overlapping each other.

    If the intended behaviour of the spec is to treat all descendants as
    if they are in the root-most stacking context, then this needs to be
    clarified.


- "With elements at the same z-index, objects are drawn in order of 
increasing z position." is very vague.

    I realize that depth sorting was intentionally left out of the spec
    to avoid requiring any particular intersection handling, but feel
    that this should probably be more clearly explained, as the 'z
    position' of a plane isn't particularly useful.


- For transitions, does it make sense to treat translatex() as 
translate3d() to reduce the number of decompositions required?

    Quite a few of the other transform functions could also be treated
    as their 3d component to increase the chances of them matching. Is
    this actually preferable?


- Matt