According to the guidelines outlined in RFC 2109 (sec. 4.2.2) and RFC 2616 (sec. 2.2, 4.2), HTTP headers are typically transmitted in ISO-8859-1 encoding. However, there is a provision for exceptions related to MIME, although such instances are rare in an HTTP context. Despite this specification, servers have the flexibility to use alternative encodings like UTF8 for cookies since they are meant to be treated as opaque data by clients. This allows for correct round-tripping of cookie data, but inconsistencies may arise if the client attempts to parse the cookie based on compliance with RFC standards.
It is advisable to refrain from including non-ASCII characters in headers whenever possible. The HTTP specifications did not adequately address this scenario, leading to complications and challenges when implementing it in real-world applications. :(