Find dead keys, stuck keys and how many you can hold at once.
Keys your system claims before the page sees them — the Windows key, Alt+Tab, Command shortcuts — cannot be tested here. That is your operating system, not a fault with the keyboard.
Press keys and they light up on the layout below. Keys you have pressed stay marked so you can sweep the whole board and see what never responded. There is also a rollover check that shows how many keys your keyboard can genuinely report at once — the thing that decides whether a game misses your inputs when you are holding several keys down.
Some keys never reach the browser at all: the Fn key is handled inside the keyboard itself, and your operating system claims a few combinations before any web page sees them. Those are limits of testing in a browser, not faults with your keyboard.
The Fn key is handled by the keyboard's own controller and never gets sent to the computer as a key at all. No web page can see it. The same is true of some media keys on certain models.
Your operating system takes those combinations first. On Windows the Windows key and Alt+Tab are claimed by the OS; on a Mac the Command-key shortcuts are. The tester blocks what it is allowed to block, but it cannot override the OS.
Almost certainly not. Most membrane keyboards report a maximum of six keys plus modifiers, which is a limit of how the keyboard reports over USB rather than a defect. Keyboards advertised as n-key rollover can report many more. If you are hitting a hard ceiling of six, that is your keyboard working as designed.
That is key blocking, and it is a property of how the keyboard is wired. Cheaper boards share wiring between keys, so certain combinations cannot be reported together. It is worth knowing which combinations your board cannot handle if you bind game controls to them.
No. Key presses are handled in your browser and never sent anywhere. Still, out of habit, do not type passwords into any key tester.
Post it in the forums and someone who has had the same fault will usually know whether it is worth cleaning, repairing or replacing. You do not need an account to read, and it is free to join.