  We develop a complete formulation of quantum gauge invariance in light-front
dynamics for interacting theories with massless vector gauge fields in the
framework of null-plane causal perturbation theory. We apply the general
results to quantum electrodynamics, showing that the so-called "gauge terms"
present in the photon commutation distribution when quantized under the
null-plane gauge condition have no contribution in the calculation of the
physical S-operator matrix elements at any order. We use this result to prove
the normalizability of the theory, and to calculate the electron's self-energy
at second order.
