User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36 Edge/15.14942 Steps to reproduce: 1. Open http://jsbin.com/huxisomisa/edit?html,css,output 2. Mouse over the red box Actual results: The custom properties are not being transitioned since the type is not known. Expected results: According to the spec, they should still be transitioned just not interpolated: "Notably, they can even be transitioned or animated, but since the UA has no way to interpret their contents, they always use the "flips at 50%" behavior that is used for any other pair of values that can’t be intelligently interpolated"