In general HDR requires an HDR display. Any given AirPlay or Cast device may or may not support these features.
For Chromium users without HDR displays, you will likely get a better looking image by manually selecting SDR in the gear menu. (It doesn’t know which to automatically choose unless I were to write a lot of fancy logic to detect what kind of display you have, and typing with paws is difficult, so I haven’t).
Once the web player ramps up to 4K quality, it is based on the same video encodes as the downloadable versions. The web player may pull from various audio tracks including AAC Stereo, Dolby AC-3, and (the default) Dolby E-AC-3, depending on what your device can do.
This is not a guarantee of what your display can do, but simply revealing which version your device is decoding. The “test” text corresponds to a change in the Dolby Vision metadata that should cause a brightness change to occur if it is being decoded.
Hopefully most Smart TVs, Plex servers, game consoles, etc. will play them happily.
The SDR version was actually derived from the HDR master using the Dolby Vision tonemapping algorithm, which optimized its downmap uniquely for each shot, and was then tweaked by hand (by paw :3) on every shot. By comparison, the YouTube SDR version is derived using a single transform (LUT) that has to apply to the whole program, and thus the YouTube SDR version doesn’t look as good in most spots.
Please reach out to Paradox Wolf if you have any questions, issues or want to give him hugs. :3 Credits for the production crew are at the end of the video. A lot of painstaking work went into this on many sides, so I hope it is appreciated. :3