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.
The shift appears to have taken place on Dec. 6, according to a Reddit thread delving into the issue. Google has been pushing the open and royalty-free VP9 codec as an alternative to the paid H.265 spec since 2014, but has never said that it would stop offering 4K video on the YouTube site in other formats, like the Apple-preferred H.264.
I’m curious what Google’s thinking is here. My guess: a subtle nudge to get more Mac users to switch from Safari to Chrome. 4K playback is going to require H.264 support if they want it to work on iOS, though.