Press any key. It lights up, and any key that has registered at least once keeps a green outline.
Hold several keys together to find your maximum. Many keyboards stop at six because of the USB spec.
You can check each key one by one to see whether it reaches the browser, and at the same time find how many keys are told apart when held together (N-key rollover) and whether certain combinations drop inputs (ghosting).
Keys the browser or OS intercepts first never reach the page - F11 (fullscreen), F12 (devtools), the Win/Cmd key, Alt+Tab and PrtSc are the usual ones. If a particular key still never responds once you exclude those, the keyboard itself may be the problem.
This page also highlights by physical key position (event.code), so the same physical key lights up regardless of your input language or layout (QWERTY, QWERTZ and so on).
Most keyboards do not read each key on its own wire - they read them through a shared matrix. In that design, certain combinations spanning the same row or column cannot be distinguished when held together, so an input is dropped: that is ghosting. Keyboards that add a diode per key can tell them all apart, and that capability is called rollover.
| Term | Meaning |
|---|---|
| 2KRO | Only two keys beyond modifiers are reliably distinguished. Common on budget boards |
| 6KRO | Six at once. The standard figure for the default USB spec |
| NKRO | Effectively unlimited. Common on gaming and mechanical keyboards |
Even an NKRO keyboard can appear capped at six over USB because of the default spec. Depending on the model you may need to enable NKRO in its software, its firmware, or with a specific key combination.
This check works only from the signals the browser receives, and is not a substitute for proper hardware diagnostics. If you want to measure how fast you can hit a key repeatedly, try the spacebar speed test or the APM test.