Checking every key
Press each key in turn. A key turns gold while you hold it and stays purple once it has worked, so any key still dark at the end did not reach the computer. Keys that are not on the drawing, such as a number pad or media keys, are listed underneath.
Ghosting and rollover
Held now counts the keys down right now, and the most so far. Press several keys together, such as a gaming combination: if a key you are holding does not light up, the keyboard cannot report that many at once. Many office keyboards manage only 2 or 3 in some combinations; gaming keyboards often manage all of them.
Keys the browser never sees
Some keys are handled by the computer before the page hears them: Fn on laptops, often the Windows or Command key, Print Screen, and media keys. Those staying dark here does not mean they are broken. Shortcuts such as Ctrl+W still close the tab, so avoid them while testing.
Questions
Does it work for laptop keyboards?
Yes. The drawing is a full-size US layout, so a laptop simply has no keys for some of it, and keys in other places still light up by what they do.
Is what I type recorded?
No. Key presses are shown on this page and go nowhere.