  Classical electrodynamics can be based on the conservation laws of electric
charge and magnetic flux. Both laws are independent of the metric and the
linear connection of spacetime. Within the framework of such a premetric
electrodynamics -- provided a local and linear constitutive law of the vacuum
is added -- the propagation of electromagnetic waves in the geometric-optics
limit can be studied. The wave vectors of the wave fronts obey a quartic
extended Fresnel equation. If one forbids birefringence in vacuum, the light
cone emerges and Maxwell-Lorentz vacuum electrodynamics can be recovered. If
minimal coupling of electrodynamics to gravity is assumed, then only the
gravitational potential, i.e., the metric of spacetime, emerges in the
constitutive law. We discuss recent results within this general framework.
