Youtube announced support for streaming HDR video in its blog today, along with a short playlist of sample videos. The streaming site supports HDR video in a variety of resolutions from 720p (1280x720) to 4K (3840x2160) and frame rates from 23.976 to 60 FPS. On that note, viewers will need an HDR display along with a playback device that supports HDR output.
Youtube specifically mentions its Chromecast Ultra as a good option to fill the role of a playback device. The Verge reports that the Xbox One S and HDR-capable Blu-Ray players can also view HDR Youtube content. Viewers without supported display equipment can still watch any video uploaded in HDR as plain SDR.
The site also published information about its requirements for uploading HDR video on its support section. The requirements page notes that software partner Blackmagic's DaVinci Resolve is currently "the only software that exports files with standards-compliant metadata out of the box." The free version of the software appears to offer limited HDR support.