
The Mute button that appears for videos in Picture in Picture Window in Chrome if you’ve Experimental Web Platform features enabled, can no longer be seen in PIP mode in Chrome 78. Francois Beaufort who works on Picture-in-Picture Web API has recently removed the Mute and “Skip Ad” buttons.
With Chrome 73 on desktop, Google has introduced Picture in Picture mode.
The feature is once enabled by selecting the option present in the right-click menu of the video, and the video then floats on top of other windows in a mini window.
In case you don’t know, the button to silence a video has been added to PIP mode in February 2019 as part of Origin Trial for the websites to test the feature without requiring to enable flag.
Users can also the get experimental Mute button in the Picture in the Picture window by turning on the “Experimental Web Platform features” flag.

Recently, a commit has been made to remove the Mute button.
The Skip Ad button was also gone as well.
The Commit made to remove it notes ” “This CL Removes the Skip Ad button in the Picture-in-Picture window and the “skipad” action from the Media Session API. It was previously added as part of the experiment.”
Though the Skip Ad button never appeared to users in development versions, the Mute button is still available and displayed in Chrome 76 in PIP mode for videos if you have got the “Experimental Web Platform features” flag enabled. This no longer the case in Chrome 78 Canary and Dev versions.
It’s not clear, whether YouTube will offer the Mute and Skip ad buttons in the future. Without these, you need to go back to the tab playing video to mute or Skip Ads.

UPDATE September 03, 2019: When we contacted Francois Beaufort, he confirmed the demise of the Mute button, but says Skip Ad is still alive in the code base, this link confirms its presence. However don’t take anything for granted, unless and until they land in Chrome stable, they’re still experimental features.
Final words:
Google has removed the Mute and Skip ad buttons from Chrome Picture in Picture mode. Though they were available as experimental in the past, they’re useful. Now users need to visit the original video by clicking the “back to the tab” option for the same.
Related articles:
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Google Chrome updates Picture in Picture UI
Chrome lets you Watch local Video files in Picture-in-Picture mode
Chrome Picture-in-Picture Mode gets Back to Video tab button