JXL currently has two levels: Level 5 (intended for browsers), allows no CMYK, limits the number of channels to 7, limits the maximum lossless bitdepth to 14-b

JXL currently has two levels: Level 5 (intended for browsers), allows no CMYK, limits the number of channels to 7, limits the maximum lossless bitdepth to 14-bit or so (the actual criterion is that the intermediate buffers can be implemented using int16_t, so the actual bitdepth limit depends on what bitdepth-widening or -narrowing transformations the encoder used), and has some more limits like that. Level 10 allows nearly anything the bitstream syntax allows, with a few "sanity check" type limits. All of the coding tools and format features in the jxl spec do have useful potential applications for the Web, except for CMYK and very high precision lossless (and this is why a decoder conforming only to level 5 does not need to handle those). E.g. layers can be useful just for compression too (e.g. a text overlay on a photo background will likely compress better / with less artifacts if it is kept in layers). The nice thing about jxl is that everything needed to display an image is specced in the codestream and handled by libjxl itself, so no need for a browser (or any other application) to implement these things themselves — and possibly not implement some of them, or not quite correctly. In avif (like in heic), a lot of codestream gaps have to be filled at the file format level (e.g. alpha), and that inevitably leads to bugs and differences between the different browsers/viewers.