I give them a mp4 + basing on my knowledge of CSS animations I put forward some suggestions on how to do it. They make fun of me and make it properly ;)

Send the exported Mac App output so that they can have a play. Brilliant feature.

Depending on what is animated i do the animation in after effects and export it by bodymovin for the lottie framework. This works best for icons or illustrative animations. So i have the full control of the animation.

Doesn't work well for screen to screen transitions.

When i create transitions (Principle/Flinto/ProtoPie) i make screenshots of the values for the easings. I also try to work with easy values. For example a transition should take a max. time of 500 ms. 250 ms are for really fast transitions. So i will describe that to my devs via a styleguide.

I use Timeline plugin and handoff animation code

If it's not complex I just give them a video file. If it's more than a simple hover, I give them specs. I present it like an animation storyboard with more details like how many pixels to the right and this kind of things.

Whenever possible, I provide them the specs, either in text form, or timeline form, so they know exactly how each elements should be animated. So far this has been working quite well and the dev team are pretty happy about it.

I covered this during my talk at Paris about Design Tools https://youtu.be/_yIsiEiBk5s?t=18m2s