Speedifier is a native Safari 10 extension that lets you control the playback speed of every HTML5 video in Safari. So you can watch your favourite videos in double speed or half the speed.
If you have an HTML5 video playing on any website just press right and select the Speedifier option you want to slow down or speed up the playback speed of the video.
On some popular pages Speedifier even adds a dedicated button to the video player to control the playback speed
Speedifier adds a button to those pages:
* Prime Video
NOTE: If a video requires an external plugin like Silverlight or Flash it is not supported by Speedifier