What a keyboard tester tells you
It lights each key you press and keeps it lit once it has worked, so a dead key is the one left dark. It also watches how keys arrive, which is where the faults people actually hit — doubled letters, blocked chords — show up.
E down 0 ms → E up 8 ms → E down 22 ms → one press, two letters: chatter, caught at 22 ms press to press
Keys are matched by KeyboardEvent.code, the physical position, not the character it types. So the test means the same in a German, French or Bangla layout, and there is no need to switch to English first.
Any board, any layout
Pick the size of the board in front of you — full, tenkeyless, 75%, 65% or 60% — and ANSI or ISO, and the drawing matches it key for key. Labels follow your own layout where the browser will read it (Chrome and Edge): a German board shows Z where a US one shows Y, and the tester names the family it read, QWERTZ, AZERTY or another script.
The Fn key is drawn but never lights: it is handled inside the keyboard and never reaches the computer. Test it through what it does — Fn with a media key shows up under Other keys, together with F13 to F24 and anything else the drawn board does not have.
Chatter: one press, two letters
A worn or dirty mechanical switch can bounce, and the keyboard reports one press as two. You notice it as doubled letters in what you type. Any key whose presses arrive closer together than the fault window — 40 ms by default — is counted as chattering and turns red.
Auto-repeat is not chatter: the extra keystrokes a held key sends are marked as repeats by the browser and ignored here. A fast trill on one key is around a hundred milliseconds between presses, well outside the window.
Rollover and ghosting
Rollover is how many keys a board reports at once. USB’s simplest mode reports six besides the modifiers, so a board that never gets past six is either a 6-key-rollover board or one running in that mode; anything above is n-key rollover.
Ghosting is the other limit. A cheap board wires its keys in a grid and cannot always tell which corner of a rectangle is down, so it blocks one key — usually in exactly the chords games use. The chords listed are ones people press; hold each, and the key that stays dark is the one blocked.
A key that never arrives cannot be detected by any page. What the tester can do is make its absence obvious, which is why each chord shows its keys lighting one by one.
Testing the keys a browser keeps
While the board has focus, every key is the test’s: Tab does not move focus, Space does not scroll, F5 does not reload, Alt does not open a menu. A few keys never reach a page at all — Ctrl+W, Ctrl+T, the Windows key — because the browser or the system acts on them first.
Capture system keys puts the tester in full screen and asks for the browser’s Keyboard Lock, which hands those keys to the page too. Chrome and Edge support it; elsewhere the button says so. Hold Escape to leave full screen. Windows still sends only the release of Print Screen, and the tester counts that release as the key working.
Every setting, and when to change it
The defaults are a full-size ANSI board with Windows labels and a 40 ms fault window. Change the board to match yours before testing, or the keys it does not have will sit in the not-tested list.
| Control | What it does | Reach for it when |
|---|---|---|
| Board size | Draws a full-size, tenkeyless, 75%, 65% or 60% board. | Always: the progress count is out of the keys on the drawing. |
| Standard | ANSI, or ISO with the L-shaped Enter and the extra key beside left Shift. | Your board was sold in the UK or most of Europe. |
| Key labels | Names the modifiers for Windows and Linux, or for a Mac. | You test a Mac keyboard and want command and option on the caps. |
| Fault window | How close two presses of one key must be to count as chatter. 15 to 100 ms. | Raise it to catch a borderline switch; lower it if fast typing is being flagged. |
| Capture system keys | Full screen plus Keyboard Lock, so Ctrl+W, Esc and the Windows key reach the test. | You need to know those keys work and you are in Chrome or Edge. |
| Report | Downloads a plain-text summary: keys tested and not, chattering keys with every doubled press, rollover and chords. | You are returning, selling or claiming a warranty on a keyboard. |
?size= ?standard= ?os= ?window= | Opens the tester on a given board, labels and fault window. | You want to send somebody straight to their own board. |
Changing the board keeps every press made so far; keys the new drawing does not have move to Other keys. Reset clears everything.
On a phone or tablet
An on-screen keyboard does not say which key was pressed — it reports the character, with no position — so there is nothing to test, and the tester says so. Connect a physical keyboard by USB-OTG or Bluetooth and it works as it does on a desktop.
When to reach for it
Before buying or selling a used keyboard, to check every key in a minute. When letters start doubling, to find which switch it is before opening the board. After cleaning or swapping switches, to see whether they all came back.
And before a game: the chords tell you whether the board will drop a key while you run, strafe and jump at the same time.