Iirc, Riva 128 only supports 8 of the 32 D3D5 blend modes, or something. Usually in the case of GPUs that don't support all blending modes the Direct3D HAL will attempt to compensate by using a different blending mode, or just give up and render opaque pixels. The results are usually pretty ugly. Riva 128 is one of the better ones for the era in this regard.