List of HTTP status codes

[1] The Internet Assigned Numbers Authority (IANA) maintains the official registry of HTTP status codes.

As the HTTP/1.0 standard did not define any 1xx status codes, servers must not[note 1] send a 1xx response to an HTTP/1.0 compliant client except under experimental conditions.

Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition.

Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has encountered an error or is otherwise incapable of performing the request.

Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and indicate whether it is a temporary or permanent condition.

[48][49] Cloudflare's reverse proxy service expands the 5xx series of errors space to signal issues with the origin server.

[51] Amazon Web Services' Elastic Load Balancing adds a few custom return codes to signal issues either with the client request or with the origin server.