FLIF can also do palettes, without arbitrary limits on the palette size. It will automatically use palettes if the image has sparse colors (and "color buckets"

FLIF can also do palettes, without arbitrary limits on the palette size. It will automatically use palettes if the image has sparse colors (and "color buckets" if it has too many colors for a palette but still relatively few). It tends to be better than PNG in terms of compression, also for palette images. I haven't looked at optimizing palette ordering yet though, so there is probably some more margin for improvement here.