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Repositories that can create a FrankenDebian if used with Debian Stable:
Debian testing release (currently stretch)
Debian unstable release (also known as sid)
Debian 8.8 is the current stable release,
https://www.debian.org/releases/The current "stable" distribution of Debian is version 8, codenamed jessie. It was initially released as version 8 on April 25th, 2015 and its latest update, version 8.8, was released on May 6th, 2017.
"guetzli" is not avalible in Debian 8. The package from sid is not installable on Debian 8 due to numerous dependency mismatches.
If you have indeed changed software sources to unstable and performed `apt install guetzli`, I guess your system is pretty much broken by now due to newer versions of libstdc++6, gcc-6 and other stuff being pulled in.
To install "guetzli" in Debian 8 you'd better build it yourself or try to backport the sid package.
Guetzli is a JPEG encoder that aims for excellent compression density at high visual quality. Guetzli-generated images are typically 20-30% smaller than images of equivalent quality generated by libjpeg. Guetzli generates only sequential (nonprogressive) JPEGs due to faster decompression speeds they offer.
I think the description should have added that it uses huge gobs of RAM while taking a long, long time to complete. So much for our modern computers being fast enough for anything. I wonder if they can leverage GPU processing into it somehow.