The MDN Web Docs site provides information about Open Web technologies including HTML, CSS, and APIs for both Web sites and progressive web apps.
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Web technologies contain long lists of jargon and abbreviations that are used in documentation and coding. This glossary provides definitions of words and abbreviations you need to know to successfully understand and build for the web.
Note: If you'd prefer a web development glossary in book form, check out The Web Development Glossary EPUB, MOBI, PDF. It's a third-party glossary that includes the entries found here, and also adds a wide range of additional entries.
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 it's 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.