Gamepad API
Also called standard mapping
The Gamepad API is how a browser reads controllers. It maps common controllers to a standard button layout, which lets one test page work with Xbox, PlayStation and many third party pads.
Browsers only expose a controller after you press a button on it. That is a privacy rule, not a detection fault.
In practice
The Gamepad API lets web pages read controllers without any drivers or downloads. It maps most common controllers to a standard layout, which is why one test works for Xbox, PlayStation and many third party pads. It doesn’t cover extras like touchpads, motion sensors or adaptive triggers.
How to check
Connect the controller and press any button with the controller test open. Browsers only reveal a controller after a button press. If the test says the layout isn’t recognized, button positions may be off. Try a wired connection or the controller’s XInput mode.