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Joe Richardson shared this little trick over on CodePen:
/* operating system font */
- If you're on Ubuntu this will be the Ubuntu Font.
- If you're on Yosemite this will be Helvetica.
- If you're on El Capitan this will be San Francisco.
- If you're on Microsoft this will be Segoe UI.
Update: Viljami Salminen has done a lot more testing on this and it looks like there are a few problems in iOS. Make sure to check out his Pen for more info.