We consider the stabilization ofMaxwell's equations with space-time variable coefficientsin a bounded region with a smooth boundaryby means of linear or nonlinear Silver–Müller boundary condition.This is based on some stability estimatesthat are obtained using the “standard" identity with multiplierand appropriate properties of the feedback.We deduce an explicit decay rate of the energy, for instanceexponential, polynomial or logarithmic decays are available for appropriatefeedbacks.