`ui-monospace` is an extended system font property [1] similar to `system-ui` that is currently only supported in Safari. This will render SF Mono on macOS and iOS [2].
It's also a generic value, like `sans-serif` that typically comes at the end of a stack. If someone has Consolas installed why show them something else only on Safari?
[1] https://caniuse.com/extended-system-fonts
[2] https://github.com/system-fonts/modern-font-stacks#monospace...