Spectral properties of two-dimensional canonical systems with locally integrable Hamiltonian are studied. We give a criterion of discreteness of the spectrum of the associated selfadjoint operator, and study asymptotic distribution of this spectrum in terms of symmetrically normed ideals of compact operators. Simultaneously, we answer a 1968 question of Louis de Branges on description of the Hamiltonians which are structure functions of some de Branges spaces.