The addition of HDR support was not mentioned in the change log of the patch, but those of you who have updated to version 13.37 of the app should see the 'HDR' lettering next to the resolution options in compatible videos. The auto quality feature will also enable HDR by default for suitable content.
HDR (high dynamic range) videos have greater contrast and offer more colors compared to standard digital video. The iPhone XS and XS Max are great candidates for viewing HDR content since they boast top-end OLED displays with high contrast ratios.
Sadly, those of you wanting to view YouTube videos with resolutions higher than Full HD (1080p) on your new iPhone will be disappointed. Most Apple devices currently don't support Google's VP9 codec that is used by the video-sharing app.
Finding HDR video on YouTube isn't so difficult. There's a plethora of such content on the video-streamer, and there are a few dedicated channels with plenty of HDR enabled clips to choose from. Still, if you need to do a quick test, feel free to check out the video below.