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In SGML, HTML and XML documents, the logical constructs known as character data and attribute values consist of sequences of characters, in which each character can manifest directly (representing itself), or can be represented by a series of characters called a character reference, of which there are two types: a numeric character reference and a character entity reference.
The SGML specification defines numerous entity types, which are distinguished by keyword qualifiers and context. An entity string value may variously consist of plain text, SGML tags, and/or references to previously defined entities. Certain entity types may also invoke external documents. Entities are called by reference.
The results of parsing can also be understood as a data tree in different notations; where the document is the root node, and entities in other notations (text, graphics) are child nodes. SGML provides apparatus for linking to and annotating external non-SGML entities. The SGML standard describes it in terms of maps and recognition modes (s9.6.
Incorrect HTML entity escaping may also open up security vulnerabilities for injection attacks such as cross-site scripting. If HTML attributes are left unquoted, certain characters, most importantly whitespace, such as space and tab, must be escaped using entities. Other languages related to HTML have their own methods of escaping characters.
Predefined named character entities are similar to internal entities: 5 of them however are treated specially in all SGML, HTML and XML parsers. These entities are a bit different from normal parsed entities, because when a named character entity reference is encountered in the document, the reference is also replaced immediately by the ...
A numeric character reference (NCR) is a common markup construct used in SGML and SGML-derived markup languages such as HTML and XML. It consists of a short sequence of characters that, in turn, represents a single character. Since WebSgml, XML and HTML 4, the code points of the Universal Character Set (UCS) of Unicode are used.
HTML 4 is an SGML application conforming to ISO 8879 – SGML. [20] April 24, 1998 ... HTML Entities; Sean B. Palmer. "Early History of HTML – 1990 to 1992".
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