Funnily enough, filling triangles pixel per pixel is so outdated that it's already misleading. In reality, all GPUs work at least on 2x2 pixel "quads", because

Funnily enough, filling triangles pixel per pixel is so outdated that it's already misleading. In reality, all GPUs work at least on 2x2 pixel "quads", because pixel shaders must be able to approximate derivatives. The approximation is done simply by taking the difference between neighbouring, simultaneously computed pixels. Of course, in practice the rasterization happens in terms of larger tiles that contain many 2x2 pixel quads, and many quads are shaded in parallel, with the shader executing in lockstep for all pixels simultaneously. But that's an optimization that can be ignored for exposition, unlike the 2x2 quads.