  Motivated by the restoration of $SU(2)\times U(1)$ at high energy, we suggest
that certain ratios of diboson differential cross sections can be used as
high-precision observables at the LHC. We rewrite leading-order diboson
partonic cross sections in a form that makes their $SU(2)\times U(1)$ and
custodial $SU(2)$ structure more explicit than in previous literature, and
identify important aspects of this structure that survive even in hadronic
cross sections. We then focus on higher-order corrections to ratios of
$\gamma\gamma$, $Z\gamma$ and $ZZ$ processes, including full
next-to-leading-order corrections and $gg$ initial-state contributions, and
argue that these ratios can likely be predicted to better than $5\%$, which
should make them useful in searches for new phenomena. The ratio of $Z\gamma$
to $\gamma\gamma$ is especially promising in the near term, due to large rates
and to exceptional cancellations of QCD-related uncertainties. We argue that
electroweak corrections are moderate in size, have small uncertainties, and can
potentially be observed in these ratios in the long run.
