What the results mean
A key stays grey when pressed
The browser received nothing. First check the obvious: is the tester focused, and does the key work in a text editor? If it works there but not here, the operating system consumed it (the Windows key and Print Screen sometimes do this). If it works nowhere, the switch is dead. On laptops, keys that need Fn will only register when Fn is held, and Fn itself is invisible to software.
A key turns red
It has been held for more than two seconds. If your finger is not on it, the key is physically stuck: pull the keycap and clean the switch, or on a membrane keyboard check for debris under the cap.
The wrong character appears in "key"
The code column shows the physical position (Backquote, KeyQ), while key shows what the operating system turned it into. If Digit2 produces " instead of @, Windows is using a UK layout on a US keyboard (or the reverse). Fix it under Settings → Time & Language → Language, or System Settings → Keyboard on a Mac.
Several keys stop registering when held together
That is ghosting or limited rollover. Most office keyboards guarantee only certain combinations of 2 to 6 keys. Gaming keyboards with N-key rollover register every key. The "max at once" counter shows what yours reached.
keyCode versus code
keyCode is the old numeric identifier (still used by some sites); code is the modern physical-key name; key is the character produced. Developers debugging shortcuts want code for position-based and key for character-based bindings.
Frequently asked questions
Why does a key not light up? +
Either the key is dead, the operating system is intercepting it, or the browser never receives it. Fn, and on some systems Print Screen and the Windows key, are handled before the browser. Try the key in a text editor as a second check.
What does "stuck key" mean? +
A key the tester sees as held down for more than two seconds. If you are not pressing it, the switch is stuck or the keycap is jammed. Clean under the cap or reseat it.
What is key rollover? +
How many keys the keyboard can register at once. Cheap keyboards manage 2 to 6, gaming keyboards report every key (N-key rollover). Hold several keys and watch the counter.
Why does the wrong character appear? +
The key works but Windows or macOS is using a different layout than the one printed on your keyboard, for example UK versus US. Change it in the keyboard or language settings.
Does the tester work on a phone or tablet? +
Only with a physical keyboard attached. On-screen keyboards do not send the same key events.