It seems that webmasters still care about files size for their web
reduce band width usage.
Today there is a better alternative than PNG for lossless images in
terms of file size.
A .bmp image compressed with lzma usually gives a smaller file size,
as you can see with this example:
9,5K — logo.png
7,2K — logo.bmp.lzma
The lzma lib is a quite small lib, and reading a .bmp images is trivial.
Making Firefox able to read this compressed format would maybe not
require too much work.
The file extension could be .bmpz or .bmz (following the .svgz).