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 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 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 XP back to Windows 3.1 it used Microsoft Sans Serif. Finally, from Windows 2.0 back to Windows 1.0 it used Fixedsys, which is not included in this set, with apologies to Windows 2.0 users.

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

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