Sometimes you just want to remove every HTML element from the YouTube page and be alone with the video you're watching. And the YouTube cinema mode doesn't give you EXACTLY what you're looking for. Indeed, it happens a lot. 

The extension is activated when its icon is clicked from a tab where a YouTube video is being played. Once clicked, the full screen mode is activated and the current window will only show the YouTube video's frame. To exit full screen mode, press F11 or Esc keys. To show all the content that has gone again, you can reload the page (press F5 or click on the reload button located at the upper left corner of the window after exiting the full screen mode). 

It may not work correctly when the extension is activated in a tab which contains a video that has been loaded and playing for a while. So, to make absolutely sure it works, it's better to reload the page, wait one or two seconds and then press the extension icon. I have to find a solution to this. 🧐

https://github.com/JMartinTorres/YouTubeOnlyVideo