System Font Stack

CSS system fonts cheatsheet.

CSS

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

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)

Source

This uses whatever system font is available.

See: System shock - Designing Medium (medium.com)