Medium -

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

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

CSS
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*/
}