Unitarity of the Cabibbo-Kobayashi-Maskawa quark mixing matrix is a mathematical expression of universality of the weak interaction across lepton and quark sectors, as well as the Standard Model’s completeness. The most precisely measured top-row elements warrant Standard Model tests at the 0.01% level. At this precision, the contribution of $V_{ub}$ is negligible, leaving one with the simple Cabibbo unitarity pattern, $|V_{ud}|=\cos\theta_C$, $|V_{us}|=\sin\theta_C$. Combining kaon decays with nuclear beta decays, currently a $2.5\sigma$ deficit is observed. This deficit, also coined the Cabibbo angle anomaly, relies on a body of high-precision experimental measurements and theoretical calculations. I give an overview of the recent developments in the SM theory of beta decays of free and bound neutrons, and kaons. I discuss future steps, as well as possible implications for BSM searches at low energies.
