System fonts

font-family: -apple-system, BlinkMacSystemFont,
    "Segoe UI", "Roboto", "Oxygen",
    "Ubuntu", "Cantarell", "Fira Sans",
    "Droid Sans", "Helvetica Neue", sans-serif;

This uses whatever system font is available. See: System shock - Designing Medium (medium.com)

Explanation

Font OS
-apple-system OS X (10.11+), iOS (9+)
BlinkMacSystemFont OS X, Chrome
Segoe UI Windows
Roboto Android 4.0+
Oxygen Linux, KDE
Ubuntu Linux, Ubuntu
Cantarell Linux, GNOME
Fira Sans Firefox OS
Droid Sans Android (until 3.2)
Helvetica Neue OS X (10.9)
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