I didn't know this worked against web fonts as well. Is there any chance you could share your webfonts config in its entirety? Maybe post it on github or some whee? I'd be hugely grateful :)
That's really the only such setting that I have :) I use DuckDuckGo as my primary search engine and that messed up i was irritating me a lot, so I was digging for a way to improve the look of Web fonts and discovered that Fontconfig can affect them.
About general idea of what all those settings are, here is a brief overview: https://en.wikipedia.org/wiki/Font_rasterization
FontConfig rules can even be applied to Web fonts, which at times can be messed up by default. Example of such rule for fixing some mess on DuckDuckGo:
The difference it made for me (note the word Wikipedia and messed up letter i):Without the rule: https://i.imgur.com/SsicEFJ.png
With the rule: https://i.imgur.com/Ehh0rZU.png