  We calculate the kaon semileptonic form factor $f_+(0)$ from lattice QCD,
working, for the first time, at the physical light-quark masses. We use gauge
configurations generated by the MILC collaboration with $N_f=2+1+1$ flavors of
sea quarks, which incorporate the effects of dynamical charm quarks as well as
those of up, down, and strange. We employ data at three lattice spacings to
extrapolate to the continuum limit. Our result, $f_+(0) = 0.9704(32)$, where
the error is the total statistical plus systematic uncertainty added in
quadrature, is the most precise determination to date. Combining our result
with the latest experimental measurements of $K$ semileptonic decays, one
obtains the Cabibbo-Kobayashi-Maskawa matrix element
$|V_{us}|=0.22290(74)(52)$, where the first error is from $f_+(0)$ and the
second one is from experiment. In the first-row test of
Cabibbo-Kobayashi-Maskawa unitarity, the error stemming from $|V_{us}|$ is now
comparable to that from $|V_{ud}|$.
