@dfogge: thanks! @TheDoc: inflexible indeed; awkward in layout (: element with clip property needs to have absolute positioning), difficult to maintain: need to set clip property values for every single thumbnail in order to determine the ‘focalpoint’. So is this worth it for only a cool hover state? Maybe not on a blogroll, but then for some it might. So I made this new codepen with a kind of ‘grid-helper image': http://codepen.io/atelierbram/pen/Ipzhi.
Work in progress.