Thinking beyond just OS X, the purpose for a general "-system-*" font and style identifier would be that web apps could more easily have the "native look" witho

Thinking beyond just OS X, the purpose for a general "-system-*" font and style identifier would be that web apps could more easily have the "native look" without having to explicitly identify the operating environment. (And identifying the operating environment still doesn't necessarily tell you what their system font is set to; that's still heavily presumptuous.) Maybe someone's Linux distro is using a serif typeface, or a monospaced serif typeface, or a monospaced sans-serif typeface, or something more like Comic Sans (have seen this more than once) as their system font. The point is, we can do better than what we have now with regard to the goal of making web applications approach the "native look and feel" more easily and more simply. It's a balance game; a balance between adding features like this (every feature adds a little bloat) that also potentially help eliminate bloat elsewhere in code (e.g., javascript to fingerprint the environment). Further, getting the system font is something that CSS should be able to know about without the aid of JavaScript; there are lots of simple forms and data entry "applications" that don't use any JavaScript (nor should they) and would benefit from a better presentation. Writing a native application for any platform, I want to get the system font, and not have to know or assume any qualities of what that font is. With typical native applications, the default font is intrinsic in the UI widgets your using, so you don't have to think about it. With the web, typically the default font is a "document font" and not the UI widget font, so you need to be able to have some sort of mechanism beyond the default font in order to get the system UI font.