I tried it with my favorite test image: https://commons.wikimedia.org/wiki/File%3ARicardo_Quaresma_%28L%29%2C_Pablo_Zabaleta_%28R%29_%E2%80%93_Portugal_vs._Arge

I tried it with my favorite test image: https://commons.wikimedia.org/wiki/File%3ARicardo_Quaresma_%28L%29%2C_Pablo_Zabaleta_%28R%29_%E2%80%93_Portugal_vs._Argentina%2C_9th_February_2011.jpg https://commons.wikimedia.org/wiki/File%3ARicardo_Quaresma_%... downsampled to 1280x853 in ImageMagick. This image is difficult for lossy compressors, because it contains some very unusual chroma signal, with the red dots on the green stripe on the red shirt. Mozjpeg does a slightly better job of preserving the dots at the almost same bitrate (as close as I can get to Guetzli quality 90), but only if you manually specify full chroma resolution. Guetzli does not allow manually choosing the chroma resolution but at least it does not totally destroy the dots by default like Mozjpeg does.