We study the regularity of propagating fronts whose motion is anisotropic. We prove that there is at most one normal direction at each point of the front; as an application, we prove that convex fronts are C^{1,1}. These results are by-products of some necessary conditions for viscosity solutions of quasilinear elliptic equations. Besides, these conditions imply some regularity for viscosity solutions of nondegenerate quasilinear elliptic equation
