Are HTTP headers case-sensitive?

HTTP header names are case-insensitive, according to RFC 2616:

4.2:

Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive.

(Field values may or may not be case-sensitive.)

If you trust the major browsers to abide by this, you're all set.

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BTW, unlike most of HTTP, methods (verbs) are case sensitive:

5.1.1 Method

The Method token indicates the method to be performed on the
resource identified by the Request-URI. The method is case-sensitive.

   Method         = "OPTIONS"                ; Section 9.2
                  | "GET"                    ; Section 9.3
                  | "HEAD"                   ; Section 9.4
                  | "POST"                   ; Section 9.5
                  | "PUT"                    ; Section 9.6
                  | "DELETE"                 ; Section 9.7
                  | "TRACE"                  ; Section 9.8
                  | "CONNECT"                ; Section 9.9
                  | extension-method
   extension-method = token

Header names are not case sensitive.

From RFC 2616 - "Hypertext Transfer Protocol -- HTTP/1.1", Section 4.2, "Message Headers":

Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive.

The updating RFC 7230 does not list any changes from RFC 2616 at this part.

tldr; both HTTP/1.1 and HTTP/2 headers are case-insensitive.

According to RFC 7230 (HTTP/1.1):

Each header field consists of a case-insensitive field name followed by a colon (":"), optional leading whitespace, the field value, and optional trailing whitespace.

https://www.rfc-editor.org/rfc/rfc7230#section-3.2

Also, RFC 7540 (HTTP/2):

Just as in HTTP/1.x, header field names are strings of ASCII
characters that are compared in a case-insensitive fashion.

https://www.rfc-editor.org/rfc/rfc7540#section-8.1.2