It appears Google recently turned on VP9 codec on YouTube for delivering 4K video. However, because of this, Safari users are unable to watch videos uploaded to the service since early December in full 4K resolution. From a report: Specifically, YouTube appears to be storing video on its servers using either the more efficient VP9 codec or the older H.264 codec. Safari only supports the latter, which explains why recently uploaded 4K videos are only able to be viewed in up to 1440p. Funnily enough, the same videos can be streamed by Safari in native 4K as long as they're embedded in another website, suggesting that the VP9 codec support requirement only applies to videos viewed directly on YouTube's website. Until Apple updates Safari to support the VP9 codec, Mac users who want to access newer 4K video on YouTube in native 2160p resolution are advised to use a different browser.John Gruber of DaringFireball writes, "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."