Hey,
in the yubikey settings page, there is a link under the title "What are yubikeys?", to the offical user guide, which is dead.
=> http://www.yubico.com/wp-content/uploads/2012/10/YubiKey-Manual-v2.2.pdf "404 Not Found"
Instead, you may link this one "https://www.yubico.com/wp-content/uploads/2015/03/YubiKeyManual_v3.4.pdf"
Or maybe, a link to the yubico documentation is better suited "https://www.yubico.com/support/documentation/", because linking to a specific version of the user guide will be obsolete soon again.
regards,
toogley/tobi