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HTTP header - Glossary | MDN 1/3 https://developer.mozilla.org/en-US/docs/Glossary/HTTP_header reference web, html, css, javascript, documentation 2026-05-05T05:33:46.179421+00:00 kb-cron

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  1. Glossary
  2. HTTP header

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 header 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:

  • Request header: Headers containing more information about the resource to be fetched or about the client itself.
  • Response header: Headers with additional information about the response, like its location or about the server itself (name, version, …).
  • Representation header: Metadata about the resource in the message body (e.g., encoding, media type, etc.).
  • Fetch metadata request header: Headers that provide information about the context in which the request is made.

A basic request with one header: Redirects have mandatory headers (Location): A typical set of headers: Note: Older versions of the specification referred to:

  • General header: Headers applying to both requests and responses but with no relation to the data eventually transmitted in the body.
  • Entity header: Headers containing more information about the body of the entity, like its content length or its MIME-type (this is a superset of what are now referred to as the Representation metadata headers)

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