The YouTube for iOS app’s most recent update adds support for High Dynamic Range (HDR) video on the iPhone XS and iPhone XS Max, However, it doesn’t offer full 4K video support.
The update to version 13.37 of the iOS YouTube app adds new options to the video quality settings, with compatible clips now showing the letters “HDR” next to the resolution to denote the high dynamic range content. HDR options can be selected manually, but the Auto quality option will also enable HDR if it is available for the currently-viewed video.
HDR offers a wider range of colors in compatible videos when displayed on an HDR-enabled device and screen, such as an Apple TV 4K and HDR 4K display. HDR offers a more vivid color picture, with less artifacts and banding then on normal displays.
The OLED displays used in the iPhone XS and iPhone XS Max are HDR-capable, and can display 4K HDR content. However, the new HDR support for the XS and Xs Max does not offer 4K support.
The YouTube app has offered HDR support for the iPhone X since last May, and the new version of the app extends that feature to the new iPhones.
The lack of 4K support for YouTube videos is the same across all Apple devices, including iPhones, iPads, Macs, and Apple TV 4k devices. The 1080p limit is likely due to Apple’s lack of operating system-level support for Google’s Vp9 codec, which is used by YouTube to stream 4K content.