YouTube
In November 2008, 720p HD support was added. At the time of the 720p launch, the YouTube player was changed from a 4:3 aspect ratio to a widescreen 16:9. With this new feature, YouTube began a switchover to H.264/MPEG-4 AVC as its default video compression format. In November 2009, 1080p HD support was added. In July 2010, YouTube announced that it had launched a range of videos in 4K format, which allows a resolution of up to 4096x3072 pixels. In June 2015, support for 8K resolution was added, with the videos playing at 7680x4320 pixels.
What did youtube change its screen format to from 4:3?
widescreen 16:9
At the time of the 720p launch, the YouTube player was changed from a 4:3 aspect ratio to a widescreen 16:9.
YouTube