Without it, I got an error that Chromium cannot be Code language: CSS ( css ) Font subsetting
To install Glyphhanger, run the following command (using -unsafe-perm=true -allow-root was necessary for me with Ubuntu on Windows 10. For the installation of fontTools (if I remember correctly) it was necessary to run Bash on Ubuntu on Windows as administrator.
Preliminary note: I installed it on Windows 10 with the Windows Subsystem for Linux. Via a whitelist, you can specify characters that should be part of the subset in any case. Besides that, it is possible to provide a font-family value, to only get the characters that are inside of elements with that rule. For that, there is also a crawler feature, so that linked pages of the website are also analyzed.
Update from July 22, 2018: Today I tried to install Glyphhanger and got an error that Chromium could not be downloaded.