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That’s not the point. This is supposed to be a stack that gets you the system font on Windows, Android, Apple platforms, and maybe Ubuntu. It looks the same way the rest of the OS does, unless the user did something weird (eg. install Avenir on Windows).

I wanted to do this but it was a lot of work. It's an open source project, though, so…

I'm guessing license issues would be a problem. Roboto wouldn't be an issue, but I'm pretty sure Microsoft's and Apple's default font files require a license to host.

I'm sure a screenshot would suffice

Yes, but they could demo the serif stack for your current OS.