Mac modifiers and what they report
A Mac keyboard has Control, Option and Command to the left of the space bar, where a Windows keyboard has Control, Windows and Alt. The browser reports them by position, not by legend, so Option arrives as AltLeft or AltRight and Command arrives as MetaLeft or MetaRight.
Command is the modifier most likely to be swallowed. macOS claims most Command combinations for itself or for the browser before a page ever sees them, which is why the Command caps are drawn dashed here and are not counted as failures.
The function row on a MacBook
On recent Macs the top row acts as brightness, volume and media controls by default, and only sends F1 to F12 when you hold the Function key. If pressing F5 changes your keyboard backlight instead of reporting F5, the key works and the row is simply in media mode.
To test the row as function keys, either hold Fn while pressing them, or turn on the setting that makes the top row behave as standard function keys in the Keyboard section of System Settings.
Butterfly and scissor keyboards
MacBooks made between roughly 2015 and 2019 used a butterfly mechanism that became notorious for keys that repeat, stick or stop responding after debris got underneath. If you are testing one of those and see a key firing twice from a single press, that is the classic symptom and it has its own page here.
Later models returned to a scissor mechanism, which fails far less often. On any Mac laptop, a single unresponsive key is far more likely to be debris than a failed switch, and it is worth trying compressed air before anything more serious.