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 4096×3072 pixels. In June 2015, support for 8K resolution was added, with the videos playing at 7680×4320 pixels.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): When was 720p HD support added to youtube?
In November 2008