The font-weight property does not get animated. This is supported in Chrome 45, Firefox 40, and Edge. I've tried via both an animation, and a transition. This is an example where font-weight should be animated: http://s.codepen.io/inclushe/debug/MaoLEw
The codepen link says "This debug view expired." Can you include an attachment on this bug that shows the problem? Thanks!
The relevant test site no longer exists. We can't take further action on this bug report.