Held nownothing
Press keys one by one and keep them held: each one should light up and the count should rise with it. Ghosting shows up here as a key that does not light while the others are held. Modern keyboards guard against ghosting by dropping the extra key rather than reporting a wrong one, so the browser never hears about it and the dark key is the evidence. Stopping at six keys plus modifiers is the USB boot protocol limit (6KRO), a design choice and not a fault; a keyboard sold as n-key rollover should light everything you can hold. Test the combinations you actually use, such as W, A, S and D with Shift and Space.