Summary: Allow authors to use a system font in a font family list.
Platform coverage: all
DevTools bug: The font panel allows authors to see the underlying font.
Other browsers: Both WebKit and Blink ship this since quite a while ago.
web-platform-tests: I added some tests for this in bug 1722488 (there
were none before) and Jonathan added some more in bug 1723552.
Security and Privacy: Note that this is not exposing more information
than before. Authors can access the same fonts via the existing system
fonts in the font shorthand. But system-ui in the font-family is a more
convenient way to access the system font family.