The Yubikey website specifies that the device registers as
a Keyboard, flash card, and flash reader when you plug it
in.
Looking at [1] it seems they make a range of products; some of their products implement multiple standards, as well as multiple USB devices. The "Premium NEO" emulates a keyboard to provide OATH HOTP, emulates a smart card reader to support PIV, and emulates a "FIDO U2F HID device" to support FIDO.
On the other hand, their "FIDO U2F Special SECURITY KEY", which only supports FIDO, does not emulate a keyboard or smart card reader - it only supports "FIDO U2F HID device"
So presumably FIDO relies on special browser support to talk to the physical hardware, and implements HID but doesn't emulate a keyboard.
On the other hand, their "FIDO U2F Special SECURITY KEY", which only supports FIDO, does not emulate a keyboard or smart card reader - it only supports "FIDO U2F HID device"
So presumably FIDO relies on special browser support to talk to the physical hardware, and implements HID but doesn't emulate a keyboard.
[1] https://www.yubico.com/products/yubikey-hardware/