System Font CSS

System Font CSS is set of @font-face rules that let you use the native system font of the OS running the browser.

body {
    font-family: system;
}

system-font.css offers eight variations of the system font family; light (300) light italic, normal (400), normal italic, medium (500), medium italic, bold (700), and bold italic.

blockquote {
    font: italic 300 system;
}

p {
    font: 400 system;
}

OSX

OSX has used three system typefaces. Since El Capitan it has used San Fransisco. In Yosemite it used Helvetica Neue. From Mavericks back to Kodiak it used Lucida Grande.

Windows

Windows has used four system typefaces. Since Vista it has used Segoe UI. In XP, it used Tahoma, which oddly enough does not have an italic variation. From Windows ME back to Windows 3.1 it used Microsoft Sans Serif. Finally, from Windows 2.0 back to Windows 1.0 it used Fixedsys. Neither Microsoft Sans Serif or Fixedsys are included in this set, with apologies.

Also, for those of opposed to joy, remember that Internet Explorer 8 does not support local @font-face rules. Therefore, should you need to reference system fonts in that browser then you will need to do so from the font declaration.

body {
    font-family: system, "Segoe UI", Tahoma;
}

Android

Android has used two system typefaces. Since Ice Cream Sandwich it has used Roboto. From Jelly Bean back to Cupcake it used Droid Sans, which also lacks an italic variation. Do you suppose OS developers dislike emphasis?

Ubuntu

Ubuntu has always used one system typeface, apty named Ubuntu. That part was easy.

Is system going to be a thing?

Maybe. There are discussions in the W3C to standardize a generic system family.