A little help along the way

Follow these steps. Every option is explained underneath. Everything happens in your browser; nothing you press is sent anywhere.

Choose your keyboardThe shape (ANSI, ISO, JIS, ABNT2, Korean or Mac) is guessed from your system; change it if it is wrong, and choose the size: full, tenkeyless, 75%, 65%, 60% or the number pad.
Click the board and press your keysEach key lights while held and keeps a tick once it has worked. The site's own shortcuts step aside while you test. Press Escape twice to leave.
Hold several at onceTo test ghosting and rollover, hold many keys together; the result keeps the most held at once.
Read the resultIt says how many keys have worked, which are still to press, the most held at once, and any key that bounced.

Every option explained

Bounces (chatter): A press within 30 ms of letting go of the same key is faster than a finger moves: almost surely a worn or dirty switch. 30 to 50 ms is possible, if you did not mean to press twice.
Repeats: A held key repeats; repeats are counted apart and never as presses, even where the browser does not say they are repeats.
Most held at once: What your keyboard, its connection and your computer allow together. Many keyboards report at most six letters at once in the simplest USB mode.
Labels: In Chrome and Edge, letter keys are labelled in your own keyboard layout; elsewhere, by the US letters at those places.
What a page cannot see: Fn, system shortcuts such as Alt+Tab, and how far a key travels never reach a web page. On a Mac a key pressed with ⌘ held sends no release.
Full screen: In Chrome and Edge, full screen lets the page catch more keys. Hold Escape to leave it.
The log: Every press, release and repeat with its code, key, location, time and the keys held, downloadable as CSV or JSON.

Keyboard tester