Source: http://lifehacker.com/fix-vertical-videos-on-youtube-with-this-bookmarklet-1634904901
You probably know that you shouldn't shoot vertical videos on your smartphone
Once you add the bookmarklet to your bookmarks toolbar, you'll be able to just click it whenever you come across a vertical video on YouTube. It will instantly take away the black space as well as enlarge the video to make it easier to view.
It's pretty cool, and a quick fix for the ever-annoying vertical issue. Hit the link below to grab the bookmarklet—bust drag and drop the button to your bookmarks toolbar..