Look inside any GIF
Drop a GIF to see every block, frame and byte in it, and anything that is broken. It is read in your browser; nothing is uploaded.
Drop a GIF here
Or open an example:
Problems
Where the bytes go
Make it smaller
Rewrites the file without changing a single pixel: unused colours, comments and metadata go, frames are cropped to the area that changes, and everything is recompressed. The new file is decoded and compared with the original frame by frame before you can download it.