the error message "unknown GIF code 0x..." is in a context (the main read loop) where only a few different codes are to be expected. I read the "grammar" in the GIF spec (Appendix B) and I believe that finding other codes in this place is a serious error, i.e. a broken GIF file.

After reading up now...you are right, this is not the place for block skipping. Thanks for your explanations!

Since we don't read "subsequent Data Streams" this is a moot point and we should exit gracefully - but I don't know yet what to do with the image object. Mark it as invalid (ERR_FORMAT) or create an image with w() = h() = d() = ld() = 0?

The GIF viewers I tested with, treat it as an invalid GIF. This seems to me also the best solution in order to discern this case from image descriptor with zero width/height.

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