In the fiddle below, I've a transition on visibility and opacity separately. The latter works but the former doesn't. Moreover, in case of visibility, the transition time is interpreted as delay on hover out. Happens in both Chrome & Firefox. Is this a bug?
This is not a bug- you can only transition on ordinal/calculable properties (an easy way of thinking of this is any property with a numeric start and end number value..though there are a few exceptions).
This is because transitions work by calculating keyframes between two values, and producing an animation by extrapolating intermediate amounts.
visibility in this case is a binary setting (visible/hidden), so once the transition duration elapses, the property simply switches state, you see this as a delay- but it can actually be seen as the final keyframe of the transition animation, with the intermediary keyframes not having been calculated (what constitutes the values between hidden/visible? Opacity? Dimension? As it is not explicit, they are not calculated).
opacity is a value setting (0-1), so keyframes can be calculated across the duration provided.
A list of transitionable (animatable) properties can be found here