Question 1

Why must semi-transparent triangles be rendered in front-to-back (or back-to-front order) to achieve proper alpha compositing?

Question 2

One of the nice properties of computing occlusion using the depth buffer (or "Z-buffer") is that the approach works just fine resolving occlusions when triangles interpenetrate. Now consider the case where the interpenetrating triangles are semi-transparent. What problem does this situation cause?