Having to adjust to the age of smartphone-shot videos where users sometimes don’t know they’re shooting in vertical (or prefer to shoot that way), YouTube is set to update its mobile app and accommodate the different formats people shoot videos these days. The upcoming update will have the software itself play the video in its appropriate size and not have it comply with a fixed area within the app. So in the future, you will see vertical, square, or horizontal videos as they were shot. You get to see a bit of that in the GIF above.
Source: TechCrunch