UserAgent: 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 the problem:
1. Open http://jsbin.com/huxisomisa/edit?html,css,output
2. Mouse over the red box

What is the expected behavior?
The animation cannot be interpolated because you don't know the type of --prop, but you should still transition the custom property following the spec and flipping at 50% between that of red and green.

What went wrong?
The custom properties are not being transitioned since the type is not known.

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 53.0.2785.143  Channel: n/a
OS Version: 10.0
Flash Version: Shockwave Flash 23.0 r0

Here is the text from the specification regarding custom properties:

"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"