If you enjoy watching videos in 4K resolution at the YouTube and uses the browser Safari, you must have realized that this option simply disappeared in the most recent videos that are available there. There is a whole discussion on Reddit about this, but I'll summarize it.
In April 2015, YouTube announced that it would start using the codec VP9 to encode your videos, making clear the reasons for not using H.264 (in practice, VP9 is a more efficient compression codec).
But what has changed from April 2015 to c? Well, apparently YouTube has become the key to this strategy and is no longer encoding new videos in H.264 (before, Google used both codecs, storing different videos and distributing them according to users' needs). Since Safari does not yet support VP9, ??browser users watched videos encoded by H.264; users of Chrome, which has supported VP9 since 2013, are routed to videos encoded with it.
As the efficiency of the VP9 is even greater in videos with 4K resolution, it seems that YouTube decided to abandon the H.264 codec once and for all (at least in this resolution). The curious thing is that this only happens on videos on YouTube; if you, for example, watch the video embedded in this post, see that the 4K option is there, available; on YouTube the maximum resolution is 1440p (using Safari, obviously).
See the difference in the images:
Of the two, one: either Apple will have to run to implement VP9 on Safari, or YouTube will resume distributing videos encoded with the H.264 codec on its website. If any company comments on the matter, we update this article.