HTML element in the context of Online rich-text editor

In information technology, header is supplemental data placed at the beginning of a block of data being stored or transmitted. In data transmission, the data following the header is sometimes called the payload or body.

It is vital that header composition follows a clear and unambiguous specification or format, to allow for parsing.

In information and communications technology, a media type, content type or MIME type is a two-part identifier for file formats and content formats. Their purpose is comparable to filename extensions and uniform type identifiers, in that they identify the intended data format. They are mainly used by technologies underpinning the Internet, and also used on Linux desktop systems.

The Internet Assigned Numbers Authority (IANA) is the official authority for the standardization and publication of these classifications. Media types were originally defined in Request for Comments RFC 2045 (MIME) Part One: Format of Internet Message Bodies (Nov 1996) in November 1996 as a part of the MIME (Multipurpose Internet Mail Extensions) specification, for denoting type of email message content and attachments; hence the original name, MIME type. Media types are also used by other internet protocols such as HTTP, document file formats such as HTML, and the XDG specifications implemented by Linux desktop environments, for similar purposes.