PiPifier is a Safari extension that lets you use HTML5 video in Picture mode.
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PiPifier is a native macOS 10.12 Safari extension that lets you use every HTML5 video in Picture mode.
If you have an HTML5 video playing on any website like YouTube, just press the PiPifier icon in Safari's toolbar to enable Picture-In-Picture for this video. Make sure you did interact with the video player once before pressing (like play/pause).
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Free