HTTP header

An HTTP header is a field of an HTTP request or response that passes additional context and metadata about the request or response. For example, a request message can use headers to indicate its preferred media formats, while a response can use headers to indicate the media format of the returned body. Headers are case-insensitive, begin at the start of a line and are immediately followed by a ‘:’ and a header-dependent value. The value finishes at the next CRLF or at the end of the message.

The HTTP and Fetch specifications refer to a number of header categories, including: