Google has announced it’s rolling out an update to YouTube for the desktop that eliminates the black bars surrounding vertical videos or those with different aspect ratios other than 16:9. For instance, 4:3 videos will now live in their own dynamic player and te UI will adjust accordingly. The same goes for 9:16 videos and other formats.
The update comes after YouTube on mobile adopted the same change.
Historically, for non wide-screen videos (not 16:9) like vertical and square videos, we would show black bars alongside the video, making the video really small. This update moves away from the need for black bars. We launched this update on mobile awhile back (both Android and iOS) so this change also aligns the desktop and mobile viewing experiences.