“But as censorship became more visible and prevalent on the Web, we started to hear from sites that they'd like to be able to make this distinction", Nottingham continues .

“More importantly, we started to hear from members of the community that they wanted to be able to discover instances of censorship in an automated fashion. For example, Lumen (previously, Chilling Effects) and Article19 expressed interest in being able to spider the Web to look for the 451 status code, so they could catalogue censorship.

“That's a use case that argues for machine-readable semantics.” And so the 451 status code became a standard.

The IETF warns that not all instances of censorship may be identified by 451 because those entities enforcing bans may also demand it is kept a secret.

But when it is used the 451 status code should “include an explanation, in the response body, of the details of the legal demand: the party making it, the applicable legislation or regulation, and what classes of person and resource it applies to.”

Bray gives as an example:

HTTP/1.1 451 Unavailable For Legal Reasons
Link: ; rel="blocked-by" Content-Type: text/html

Unavailable For Legal Reasons
This request may not be serviced in the Roman Province of Judea due to the Lex Julia Majestatis, which disallows access to resources hosted on servers deemed to be operated by the People's Front of Judea.