> Analytics tracking pixels normally ARE GIFs, specifically a 43 byte transparent 1x1 GIF.

Is there a reason for this?

And no, a GIF is a GIF, nothing else is a GIF, an mp4 is not a gif, webm is not a gif.

Are you asking why a GIF is used, or why the 43 bytes are needed?

I think a GIF is likely the smallest wide-supported resources format (force the browser request a URL to load a specific resource). The 43 bytes is the minimum size for a valid GIF, so the browser won't log an error loading the image and go through all load events.

It looks like in modern browsers it's 35 bytes: https://stackoverflow.com/a/15960901/407650

you can say that, but the big gif hosting serve other formats with better compression by default.