If a website declares a CSS @font-face rule that defines font-family foo, and then later uses `font-family: foo` in a style; and foo is also the name of a system font-family; do browsers then skip the @font-face "version" of the font and use the system one instead? Hopefully not bothering to even download the web font?

I've never run into this case, despite using web fonts a couple of times, because I don't think I've ever seen anyone attempting to use a font that's a system font of an OS as a web font.

But that seems like the main use-case here: knowing that at least macOS ships with Proxima Nova, you'd declare a dependency on the Google Fonts copy of Proxima Nova as a polyfill of sorts; and then the OSes that have it would use their version instead of needing to pull it from Google.