The propertyName in the event is empty if the transition-property is set to all (or left as the default). If you transition multiple properties it is impossible to tell which event is firing for which property. You can see this, for instance, in LayoutTests/transitions/transition-end-event-all-properties.html. It logs an empty propertyName for each event.
Created attachment 23509 [details]
Patch, including modified layout test
r36774 = 0f83a518dda6b9973d30f2bae7a1b82083bc5b29 (trunk)