Transitions/animations on grid-template-columns|rows are not working on WebKit, neither on Chromium.
They only work on Firefox so far.
At some point we should add support for that on WebKit too.
There are a bunch of tests under http://w3c-test.org/css/css-grid/animation/ regarding that.
It's worth noting that repeat() needs to be handled carefully:
> If two repeat() notations that have the same first argument (repetition count) and the same number of tracks in their second argument (the track listing), they are combined by combining each component of their computed track lists by computed value (just like combining a top-level track list). They otherwise combine discretely.
But this is not currently possible, since non-auto repeat() is expanded at computed-value time (bug 202259).