The key takeaway:

  > New stacking contexts can be formed [… w]hen an element has an 
  > opacity value less than 1.

I had no idea that was the case. The CSS 2.1 spec [1] notes that "other properties may introduce stacking contexts, for example 'opacity'". The CSS 3 spec [2] elaborates:

  > Since an element with opacity less than 1 is composited from a
  > single offscreen image, content outside of it cannot be layered
  > in z-order between pieces of content inside of it.

[1]: http://www.w3.org/TR/CSS2/visuren.html#z-index [2]: http://www.w3.org/TR/2010/PR-css3-color-20101028/#transparen...