Just found the paper with this lossless coding proposal linked from the wikipedia page: http://jpegclub.org/temp/JPEG_9_Lossless_Coding.doc
It has some compression ratio comparison tables, showcasing libjpeg-9 lossless coding and allegedly outperforming the competition. I decided to also give WebP a try (using libwebp-0.2.0.tar.gz):
The very first file from the set compresses to 504672 bytes with WebP, which seems to be significantly better than 574K reported for libjpeg-9 using the same file. I have not run any tests through the rest of the files, this might be a good exercise for somebody else
$ wget http://www.r0k.us/graphics/kodak/kodak/kodim01.png
$ cwebp -m 6 -lossless kodim01.png -o kodim01.webp
$ ls -l kodim01.webp
-rw-r--r-- 1 ssvb ssvb 504672 Jan 14 23:17 kodim01.webp
In any case, the paper says: "In April/May 2012, a new feature was found and implemented in the IJG software which significantly improves the lossless compression of continuous-tone color images, outperforming other currently popular methods and thus making the new feature very attractive for practical application. A development version with the new feature is currently presented by InfAI Leipzig and IJG, and is planned for release as a new major IJG version 9 in January 2013."
But for a new lossless image compression method developed in 2012/2013, totally ignoring WebP and maybe some other modern codecs seems to be a bit unfair, isn't it?
Just let the aging JPEG and PNG formats keep providing best compatibility with the existing and future software. That's their best feature today and they really have nothing else to offer.