If you are like me, you like using as much of your screen to watch videos on YouTube. The comments, title bar, and the rest of the edge of the screen takes away the possibility of looking at all those 1080p pixels available now.
To make any video fill your browser window, drag the link that says YT Full below to your bookmarks toolbar. You can also right click and save to bookmarks. Then, while on a YouTube video page, click on it. The page will be replaced by the URL of the embedding version of the video, filling the viewport.
Inside the bookmarklet you will see this:
It’s a short script that takes the current URL from the address bar, yanks the video ID from it, and replaces the tab URL with the embedded version URL.