I will give a brief overview of the Standard Model theory inputs needed for the precise determination of the Cabibbo-Kobayashi-Maskawa matrix element $V_{us}$ from semileptonic kaon decays, focusing on the long-distance electromagnetic corrections. I will then describe our recent effort to pin down this correction at sub-permille level, which further sharpens the so-called ``Cabibbo angle anomaly'', an interesting observation which may point towards new physics.