Why quality slider only 85 to 100
Executing Guetzli with a lower quality setting than 85, will result in the following :
Guetzli should be called with quality >= 84, otherwise the output will have noticeable artifacts. If you want to proceed anyway, please edit the source code. Guetzli processing failed
some nice answer:
Guetzli algorithm is beneficial only for this quality or higher. If you work around it and lower the quality, you'll only waste your time generating average JPEGs, without Guetzli improvement.
If you need even smaller files and can tolerate bigger distortions, MozJPEG is a better choice for quality < 80.