This is a very modern CSS only button where you have a small preview of the video playing in a circle and when you hover over it the preview expands to a bigger circle revealing more of the video and on click the video is loaded full screen. It was designed by Milan Raring.
If you are having trouble with the pen, try the archived copy on GitHub
See the Pen
Video button animation – Only CSS by Milan Raring (@milanraring)
on CodePen.