Error 451 makes it clear when a website has been blocked for legal reasons.
When people visit a page that's blocked for legal reasons, it can be hard to work out why that page isn't accessible.
Sometimes a 403 Forbidden message appears when a court has ordered a website to be blocked. 403 Forbidden is supposed to appear when a website wants to refuse access to somebody.
403 Forbidden shouldn't be shown when a site is blocked for legal reasons. That's not what it's meant for.
People should know that 403 Forbidden only means that the site wants to deny them access.
Error 451 would let people know that a court ordered the site to be blocked. This might be because it contains restricted political content or carries copyright material.
Some ISPs, such as Virgin Media in the UK, tell their customers that a court has ordered them to block a website.
It's still difficult to work out precisely why a website is blocked and to check if it's been blocked by mistake.
A really good Error 451 message would tell their customers how to challenge a block, how long the block's expected to last, where the relevant legal documents are and which legal authority imposed the blocking order.