https://www.cnet.com/news/youtube-hdr-comes-to-iphone-x/

YouTube has rolled out high dynamic range (HDR) to users of the iPhone X, enabling improved contrast and color on HDR-enabled YouTube videos.

As spotted by Macrumors, videos like those from the popular HDR Channel, are now able to be viewed in HDR mode. Neither the iPhone X nor the Apple TV are able to output the videos in 4K, however.