csswizardry/csswizardry-grids — pull request #41 by jpcarmona (2013-08-23 11:33:21+00:00)
Title: Using em instead of px because of zoom

An old article but it explains the problem of using px instead of em.
http://blog.cloudfour.com/the-ems-have-it-proportional-media-queries-ftw/