I'm not convinced that FLIF can really do that. One file for all versions is a nice idea, but in practice, the first 20KB of a progressive FLIF is often an inferior substitute for a 20KB JPEG, to say nothing of BPG.
Note that the image in the example was chosen specifically to highlight FLIF's strengths, but the math shows that you could combine the uninterlaced FLIF with the 17KB JPEG 2000 and still take up less space than the interlaced FLIF.
FLIF looks like a very impressive lossless codec indeed, and even is competitive when used as a lossy codec, but the quality of partially downloaded interlaced files just isn't competitive with lossy codecs--including lossy FLIF.
I agree: if you're fine with lossy, then it's hard to beat lossy formats with a lossless format.
Maybe in some future version of FLIF, with some DCT/DWT-like transform and an option to postpone least significant bits until the end of the bitstream, we can truly get there. But at the moment we're not there yet.
See: http://flif.info/example.html
Note that the image in the example was chosen specifically to highlight FLIF's strengths, but the math shows that you could combine the uninterlaced FLIF with the 17KB JPEG 2000 and still take up less space than the interlaced FLIF.
FLIF looks like a very impressive lossless codec indeed, and even is competitive when used as a lossy codec, but the quality of partially downloaded interlaced files just isn't competitive with lossy codecs--including lossy FLIF.