Sure, but usually in any file format, an encoder has some choice in how to produce its output, with no functional difference either way. For instance, different PNG encoders might include the necessary chunks in a different order in the file. And my concern is that some such choices, if they align with choices described in the copyrighted spec, might be protectable as a creative arrangement or similar. (E.g., a spec might list the chunks in such an order that their names form a creative acronym.)

Of course, the defense against this would be to scramble (or normalize) all non-functional choices compared to anything in the spec. But you have to be careful to make sure there's nothing left of the spec's non-functional influence.

At least Oracle v. Google appears to provide some ammunition here in favor of implementers: the court found the transformative use in that case enough to trump even the byte-for-byte copying of the API signatures. So perhaps interoperability could similarly trump such non-functional copying from the spec. But overall, it's still on shakier ground than I'd like.