Stairs are made out of two cuboids: one for the lower slab portion, and another for the upper quarter of a block section. Culling, however, is solely based on the whole block's collision box, causing some quads to still be rendered in situations like the following, when they are fully obscured:

In these situations, we could probably get rid of some more faces:

The minecraft code is not built in a way that would make this easy. I also don't think there would be enough instances of a match like this happening. to justify the cost of that many more checks. So ill put this on the back burner