Bray’s new Error 451 would work somewhat like the Error 404 pages you’ve probably seen. But instead of telling you that the page could not be found, an Error 451 response would let you know that the page you were looking for had been censored.
As it stands, most web-blocking tools return a 403 error (which means access is forbidden) when denying access to censored pages. For instance, UK ISPs, which are now required to block The Pirate Bay, typically return a 403 error code when doing so.
The main advantage of the proposed 451 code is that it would add an explanation of why the content was unavailable. “Responses using this status code should include an explanation, in the response body, of the details of the legal restriction,” writes Bray in his proposal. Details would include tidbits like which legal authority is imposing the restriction, and what class of resources it applies to.
That would mean ISPs could return a message absolving themselves and letting citizens know that the government, not the ISP, is censoring the web.
Bray notes in the proposal that many governments might not want such censorship transparency and would likely take steps to prevent it. As such the 451 status code would be optional and clients (like your web browser) are instructed not to rely upon its use. It also remains to be seen whether the Internet Engineering Task Force, which oversees standards like HTTP error codes, will approve of the idea.