  Leptonic and semileptonic kaon decays represent till now the golden modes for
the extraction of the Cabibbo angle from experiments, provided the relevant
hadronic quantities, namely the ratio of the kaon and pion leptonic decay
constants, $f_K / f_\pi$, and the semileptonic vector form factor at zero
four-momentum transfer, $f_+(0)$, are computed accurately from QCD. In the last
years, using large-scale lattice QCD simulations, the determination of both
$f_K / f_\pi$ and $f_+(0)$ has reached an impressive level of precision
($\approx 0.3 \%$), which is expected to be further improved in the near
future. However, at a permille level of precision both electromagnetic and
strong isospin-breaking effects cannot be neglected anymore. In this
contribution a new lattice strategy aiming at determining QCD+QED effects in
the inclusive leptonic decay rates of charged pseudoscalar mesons is briefly
illustrated, and the preliminary results obtained in the case of $\pi_{\ell 2}$
and $K_{\ell 2}$ decays are presented. As for the semileptonic $K_{\ell 3}$
decays, it is pointed out that the determination of the relevant form factors
in the full kinematical range covered by the experiments is necessary for a
more stringent test of the Standard Model. In this respect the perspectives
based on the only existing results from the European Twisted Mass Collaboration
(ETMC) are quite encouraging.
