drop-shadow() is not implemented on composited layers. See for example css3/filters/effect-drop-shadow-hw.html, or http://test-results.appspot.com/dashboards/flakiness_dashboard.html#showExpectations=true&tests=effect-drop-shadow-hw
Created attachment 128528 [details] Patch
Comment on attachment 128528 [details] Patch Cool!
Committed r108674: <http://trac.webkit.org/changeset/108674>