[tatersolid]

>This only includes fonts already present as standard font on the operating system, so presumably they are all permissively licensed.

No they’re not. Apple, Microsoft and even some Google systems include fonts which are commercially licensed and usable only on a device licensed for that OS. In fact Apple and Microsoft contracted the development of many fonts you see on today’s devices and the web.

[aprilnya]

But you’re not distributing the font. You’re not doing anything bad. “font-family: ‘Copyrighted Font’” is not distributing the font, and so is not copyright infringement, you’re just telling the browser to use it if it’s already there.

Disclaimer: Not legal advice

[tatersolid]

You’re correct that so long as you don’t serve or otherwise distribute the font files or a derivation the licensing doesn’t matter. But my comment specifically referred to the “they are all permissively licensed” portion which I quoted.

San Francisco, Segoe, and many other OS-included fonts have commercial proprietary licenses, not permissive licenses as the GGP post claimed. Try to serve San Francisco WOFF2 files as a font-family src on your site and see how Apple’s lawyers react.