[css3-3d-transforms] Formal definition of 'backface-visibility'

http://dev.w3.org/csswg/css3-3d-transforms/#backface-visibility-property
doesn't have any formal definition of when the "back" of something
is visible.

For example, some obvious possible definitions would be:
 * if the element's transform involves projecting the vector [0 0 1]
   to a result with a negative Z component
 * if the element's cross product of the transform of [1 0 0] and
   the transform of [0 1 0] gives a result with a negative Z
   component
These two give different results in some cases (I think when the
transform matrix has a negative determinant).

Matt Woodrow implemented the latter in Gecko in
https://bugzilla.mozilla.org/show_bug.cgi?id=505115 ; I'm not sure
if that was for compatibility with WebKit.

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla Corporation               http://www.mozilla.com/   𝄂