Checking that every key works
A sticky, dead or double-typing key is one of the most common keyboard faults, especially after a spill or on second-hand laptops. This tester draws a standard keyboard layout and lights up each key as you press it, using the KeyboardEvent.code value, which identifies the physical key regardless of language or layout. Keys you have tested stay marked, so you can work across the keyboard and immediately spot one that never lights up. The details line shows the key value (the character produced, which depends on your layout), the code, the legacy keyCode number and the location, which tells left and right Shift, Ctrl and Alt apart.
Ghosting and rollover
Many keyboards cannot report every combination of keys pressed at the same time. Membrane and laptop keyboards often handle only certain combinations, so pressing three or four keys together may drop one (blocking) or report a key you did not press (ghosting). Gaming keyboards advertise n-key rollover, meaning every key is read independently. Hold down several keys, for example W, A, S, D and Space, and watch the “most keys held at once” counter. If it stops increasing before you run out of fingers, you have found your keyboard's limit for that combination.
Keys the browser cannot see
Some keys are handled by the operating system or the keyboard itself and never reach a web page. The Fn key, many media and brightness keys, Print Screen on some systems and the Windows key in certain configurations will not register. Browser shortcuts such as Ctrl + W or Ctrl + T are also reserved for safety, so the tester does not block them. Arrow keys, Space and Tab are blocked by default inside the test area so the page does not scroll; tick the checkbox if you want them to behave normally.
Repeating keys and chatter
If a key types a letter twice when pressed once (switch chatter), you will see two events in quick succession. Holding a key shows “repeating” once the system auto-repeat kicks in, which is normal. Nothing you type is recorded or sent anywhere.