High dynamic range video (HDR) offers many benefits, like a much broader range of color and luminance compared to standard dynamic range video (SDR).
How to watch YouTube in HDR on iOS
Do the following to watch YouTube in HDR on your iPhone X.
1) Open the YouTube app on your iPhone X.
TIP: Be sure that you’re using the latest version of the app.
2) Tap a video you’d like to watch.
3) Tap the video to reveal onscreen controls, then hit the three dots in the upper-right corner.
4) Choose Quality from the popup menu.
5) Choose one of the resolutions that include “HDR”