JPEG XL

@jpegxl Will #jpegXL need additional metadata when used with HTML's srcset?
While the format's progressive rendering can do away with the need to have different files per resolution, browsers will instead need to know how much of the image they'll fetch. How would we pass that information to the browser?
Extra syntax in srcset, eg p.jxl, p.jxl \<2040 100w, p.jxl \<6974 200w to tell that for a 100px wide view the browser needs to load the first 2040 bytes? Fragment identifier tricks?