So all these years I thought a GIF image is limited to a maximum of 256 colors. This nice article (in czech :/) explains there is something called local palettes.
Instead of having a global image palette, you can define a palette per frame. So it is possible to ‘assemble’ an image out of all these 256 color-using tiles.
Some software, such as firefox apparently doesn’t interp[ret the 0 milisicond delay properly so you get a nice ZX Spectrum like load effect :) (perhaps a CPU safety feature).