What appears to be Google's shift to the VP9 codec for delivering 4K video on the YouTube homepage is preventing Safari users from watching videos uploaded to the service since early December in full 4K resolution, but not from viewing webpage-embedded videos in the same resolution.

Videos uploaded to the service prior to Dec. 6 in 4K resolution can still play back in full 4K resolution on Safari from the YouTube homepage. Additionally, Mac users utilizing Chrome still have the ability to play back new videos in 4K, as Safari is the only holdout among the major browsers that doesn't support the codec.

However, further confusing the issue, a new video embedded in a webpage such as the 4K video of the Apple Campus flyover from Wednesday, can still be played back in the embed in 4K. Shifting to the YouTube homepage to view the video reverts to a maximum 1440p resolution.

The issue manifests on all Apple hardware, regardless of OS revision, or Safari version.

Source: appleinsider