Currently 18181-2 says the jxlp (partial codestream) boxes shall be in order, which is what decoders are assuming. If this is not the case, then jxl-rs will produce an error message "Out-of-order jxlp box found", which is good.

In the future there might be a relaxation that allows out-of-order jxlp boxes. This means there will likely be a need to handle the case of out-of-order boxes. The existing syntax where in-order is obligatory will remain the default; it hasn't been decided yet how it will be signalled that the relaxed syntax is allowed but it would make sense to signal this in the ftyp box.

See also: libjxl/libjxl#4735