Here you can see the mobile share page HTML. In the clip the device is rotated from portrait to landscape mode. Videos can play 'inline' for devices that support it. Also, you can see if the video doesn't fit in the viewport, the user can scroll, or resize-to-fit using the bottom-right icon.
(For video controls, pause, reverse, etc., view on: http://gfycat.com/WhisperedTestyArmadillo)