Not to be confused with, high dynamic range for video and TVs expands the range of both contrast and color significantly. Bright parts of the image can get much brighter, so the image seems to have more "depth." Colors get expanded to show more bright blues, greens, reds and everything in between. OLED-powered screens like the one on the iPhone X are particularly adept at showing HDR.
The HDR capability isn't stated in the release notes for the latest 13.17 update which only lists "General fixes and stability improvements".
Representatives for YouTube did not immediately respond to CNET's request for comment.