Understand Mac top-row media keys versus F1–F12
Brightness, volume and other top-row controls often perform system actions. Standard F1–F12 events may require holding Fn / Globe, depending on your keyboard and settings. Models differ, so check the symbols on your own keyboard.
Check the function-key setting in macOS
Open System Settings and inspect Keyboard settings, including Keyboard Shortcuts and Function Keys where available. The option to use F1, F2 and so on as standard function keys changes which action requires Fn. The exact location varies with macOS version.
Test F1–F12 events with the Mac keyboard checker
Focus the tester, press an F-key and watch the event log. Try the Fn-modified version if the system performs a media action. Some keys also have browser or system assignments that prevent the page from seeing them.
Why Fn, Touch ID and power are not normal browser keys
Fn / Globe may modify another key without producing its own event. Touch ID and power functions are handled outside ordinary web keyboard input. A missing highlight for these controls is not a reliable failure indicator.