In the future, watching UltraHD on YouTube will require support for Google’s VP9 codec, which Apple has so far not supported. For a decade, content providers have used MPEG4 (and its sub-classes such as AVC / H.264) for streaming services. With the transition to UHD, new formats have emerged. HEVC was created by the same group who created MPEG4 but Google has developed its VP9 format and is promoting it as a de facto standard. Google is using it for its own services such as YouTube whereas Apple prefers MPEG4. VP9 is royalty-free whereas MPEG and HEVC are not.
In the past, YouTube has included 4K videos in both MPEG4 and VP9 and this has allowed Safari users to watch using MPEG4. Now this will no longer be possible as YouTube is switching to serving UHD via VP9 exclusively for new videos.
The change means that owners of iMac 4K and iMac 5K will have to switch to using Google Chrome in order to watch YouTube in 4K in the future. Apple has so far refused to back Google’s VP9 format as well as the predecessor VP8. The feud goes back many years to when Steve Jobs was alive and he once indicated that he thought that VP8 infringed on MPEG4 patents.
An alliance consisting of Amazon, AMD, ARM, Cisco, Google, Intel, Microsoft, Nvidia, Mozilla, and Netflix is working on creating a next-generation video format. Apple is not currently listed as a member of the alliance. (Alliance for Open Media Established to Deliver Next-Generation Open Media Formats)