Stuart Frisby documents that you shouldn’t use the font shorthand when using a System Font Stack:
…don’t use -apple-system at the head of a shorthand font declaration, and test thoroughly, especially when playing around with proprietary stuff like system font declarations. If it looks like a vendor prefix and smells like a vendor prefix, chances are at least one browser is going to treat it like a vendor prefix.
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Implementing system fonts on Booking.com — A lesson learned is a post from CSS-Tricks