Is there any legitimate way to use Apple's new system font, San Francisco, in non-Apple systems? Do other platforms support a generic "system" font family?
On platforms which do not support “-apple-system,” the browser will simply fall back to the next item in the font-family fallback list. This provides a great way to make sure all your users get a great experience, regardless of which platform they are using.
There are currently discussions in the w3c regarding standardizing this value so authors could simply specify “system.” However, these discussions have not reached consensus, so WebKit prefixes this value.
* the console font
* one of the various "fixed" fonts that come with X11
* any of the default X11 fonts
None of which are likely to be used as the default for title bars, menus, etc.
Graphical distros have a standard font in their UI.
Oh well. Something else to roll into systemd!