YouTube recently began rolling out support for high dynamic range video content on iPhone X, as noted by users on the MacRumors forums and Reddit. HDR videos provide a broader range of colors and luminance compared to sub-HDR content, and the format also supports quality video playback on a wide variety of screen sizes.
For iOS users, YouTube videos in HDR can be viewed on the iPhone X, but the feature does not yet appear to be working on the latest iPad Pro models. YouTube has many HDR-supported videos on its site, with dedicated channels like The HDR Channel providing videos for you to watch in HDR on iOS. To make sure of the quality of playback find a video to play, then tap anywhere on the video itself, tap the three dots in the top right-hand corner of the screen, tap "Quality," and then make sure it reads 1080p60 HDR.
Apple debuted HDR support last year with the "Super Retina" display in the iPhone X, which became the first HDR OLED display incorporated into one of Apple's smartphones. Apple also lists the iPhone 8 and iPhone 8 Plus as supporting HDR, but the displays for these devices are not the same as the Super Retina display on iPhone X, so any HDR content played on the 8 or 8 Plus isn't true HDR.
On the iTunes Movies Store on Apple TV 4K, select films also support 4K, HDR10, and Dolby Vision when paired with compatible 4K television sets. Although YouTube recently updated its Apple TV app and listed a few videos as "4K," the app still has yet to support true 4K. When viewing details about a video, maximum available resolution is still listed as 1920 x 1080, while 4K resolution is 3840 x 2160.