L²-error estimates for finite element approximations of boundary fluxes

We prove quasi-optimal a priori error estimates for finite element approximations of boundary normal fluxes in the $L^2$-norm. Our results are valid for a variety of different schemes for weakly enforcing Dirichlet boundary conditions including Nitsche's method, and Lagrange multiplier methods. The proof is based on an error representation formula that is derived by using a discrete dual problem with $L^2$-Dirichlet boundary data and combines a weighted discrete stability estimate for the dual problem with anisotropic interpolation estimates in the boundary zone.