Note that XYB JPEG isn't the only way to increase the effective bit depth of the image. Using a typical color space but using a different transfer function such

Note that XYB JPEG isn't the only way to increase the effective bit depth of the image. Using a typical color space but using a different transfer function such as Perceptual Quantizer or Hybrid Log-Gamma has a similar effect, and this is exploited right now. Take an HDR JXL file and tell djxl to decode it to a jpeg or a png, and you'll get a PQ-encoded file with an ICC profile containing the appropriate information for HDR rendering and for tone-mapped SDR rendering :)