alt Hacker News> and generally its HDR support is much better than AVIF
Not anymore. JPEG had the best HDR support with ISO 21496-1 weirdly enough, but AVIF also just recently got that capability with 1.2 ( https://aomedia.org/blog%20posts/Libavif-Improves-Support-fo... ).
The last discussion in libjxl about this was seemingly taking the stance it wasn't necessary since JXL has "native HDR" which completely fails to understand the problem space entirely.
The JXL spec already has gainmaps...
Also, just because there's a spec for using gainmaps with JPEG doesn't mean that it works well. With only 8 bits of precision, it really sucks for HDR, gainmap or no gainmap. You just get too much banding. JXL otoh is completely immune to banding, with or without gainmaps.