This is a follow-up of bug 103677. We currently don't test this case as we allow positioned elements to be grid items. I proposed a specification change [1] that would disallow this case and would thus change our behavior. Once this is migrated to the specification for good, we can add a test to ensure we properly resolve percentage paddings / margins. [1] http://lists.w3.org/Archives/Public/www-style/2012Nov/0546.html