In the vertex shader, we determine the front side of the polygon by the direction of the normal vector, and in the fragment shader, the determination is based on the polygon's winding. For this to work properly, the normal vector must be defined appropriately for the face determined by the setting of glFrontFace.
An alternative choice for this recipe would be to determine whether the face being shaded is a front or back face first in the vertex shader, and send only a single result to the fragment shader. One way to do this would be to compute the dot product between a vector pointing towards the camera (the origin in camera coordinates), and the normal. If the dot product is negative, then the normal must be pointing away ...
