“…The additional complexity in comparison to the scalar-valued case begins with the correct definition of tangential and normal boundary conditions in a setting of low regularity [52,12,13,53,27]. When non-mixed boundary conditions are imposed, so that either Γ T = ∅ or Γ T = ∂Ω, then Rellich-type compact embeddings H(div, Ω, Γ N ) ∩ H(curl, Ω, Γ T ) → L 2 (Ω, R 3 ) and vector-valued Poincaré-Friedrichs inequalities have been known for a long time [51,47,55,17].…”