Posté par: igor51 le décembre 23, 2015, 14:00:32
It's meant to let you know that you're not seeing what you want to see because a site's been blocked for legal reasons, be it censorship or otherwise.
Source: Welcome to HTTP error code 451: Unavailable for legal reasons (http://feedproxy.google.com/~r/nakedsecurity/~3/Iz-BEBDP68Y/)