Keyboard tests

A keyboard fails in three broadly different ways, and each has its own test. A key that never registers is a dead switch, debris under the keycap, or a break in the matrix — the keyboard test finds those one key at a time. A key that works alone but drops out while you hold others is ghosting, which is a limit of how the board scans its matrix rather than a fault; the key rollover test shows where that limit sits.

The third failure is timing rather than detection: keys that repeat, stick, or arrive doubled. Those show up as duplicated characters while typing, and the spacebar counter is a quick way to see whether one press is being reported once or twice.

All three read the browser's raw keyboard events, so they report the physical key your board sent rather than the character your layout produced. That makes them layout-independent, but it also means keys the operating system intercepts first — Alt+Tab, the Windows key, Print Screen on some systems — may never reach the page.