Font Stack

Font	Device Targeted
-apple-system (San Francisco)	iOS Safari, macOS Safari, macOS Firefox
BlinkMacSystemFont (San Francisco)	macOS Chrome
Segoe UI	Windows
Roboto	Android, Chrome OS
Oxygen / Oxygen-Sans	KDE
Fira Sans	Firefox OS
Droid Sans	Older versions of Android
Ubuntu	Ubuntu
Cantarell	GNOME
Helvetica Neue	macOS versions < 10.11
Arial	Any
sans-serif	Any

Examples

Wordpress 4.6:

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

Medium:

body {  
  font-family: -apple-system, 
               BlinkMacSystemFont, 
               "Segoe UI", 
               Roboto, 
               Oxygen,
               Ubuntu,
               Cantarell,
               "Open Sans",
               "Helvetica Neue", 
               sans-serif;
}

GitHub:

body {  
  font-family: -apple-system, 
               BlinkMacSystemFont, 
               "Segoe UI", 
               Roboto, 
               Helvetica,
               Arial,
               sans-serif,
               "Apple Color Emoji",  /* Emojis*/
               "Segoe UI Emoji", /* Emojis*/
               "Segoe UI Symbol"; /* Emojis*/
}