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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.
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.
The format is the same as for any entity reference: &name; where name is the case-sensitive name of the entity. The semicolon is required. Because numbers are harder for humans to remember than names, character entity references are most often written by humans, while numeric character references are most often produced by computer programs. [1]
External entities are not supported in basic profiles for SGML or in HTML documents, but are valid in full implementations of SGML and in XML 1.0 or 1.1 (including XHTML and SVG, even if they are not strictly needed in those document types). An example of internal entity declarations (here in an internal DTD subset of an SGML document) is:
Some cheat sheets show 3 digit references, some show 4 digit references. If I'm correct, the 3 digit references refer to ISO-8859-1 and the 4 digit references refer to ISO10646/Unicode. For example, I'd like to use an en dash on my site, but I'm not sure whether to use – or –…
HTML markup consists of several key components, including those called tags (and their attributes), character-based data types, character references and entity references. HTML tags most commonly come in pairs like < h1 > and </ h1 >, although some represent empty elements and so are unpaired, for example < img >.
Character entities can be included in an HTML document via the use of entity references, which take the form &EntityName;, where EntityName is the name of the entity. For example, —, much like — or —, represents U+2014: the em dash character "—" even if the character encoding used doesn't contain that character.
A general entity can only be referenced within the document content. A parameter entity can only be referenced within the document type definition (DTD). Entities are also further classified as parsed or unparsed: A parsed entity contains text, which will be incorporated into the document and parsed if the entity is referenced. A parameter ...