I'm more wondering why they use css* to rotate the image, instead of just using a sprite with both triangles and clipping.

Granted, on a site as simple as HN this makes no difference at all. But I think there is an overall trend to push more work onto the client, which kinda makes sense since they have more and more CPU and RAM over time. OTOH, its bad because I can throw away my old "office" desktop and my "netbook" (it has net in the name!) because they can't browse the average webpage anymore!