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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.
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 >.
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.
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 whitespace character is a character data element that represents white space when text is rendered for display by a computer.. For example, a space character (U+0020 SPACE, ASCII 32) represents blank space such as a word divider in a Western script.
In web page design, and generally for all markup languages such as SGML, HTML, and XML, a well-formed element is one that is either a) opened and subsequently closed, or b) an empty element, which in that case must be terminated; and in either case which is properly nested so that it does not overlap with other elements.
An empty element may be: An empty HTML element, one with tag(s) but no content (HTML element § Empty element) An empty XML element, one with tag(s) but no content (XML § Key terminology) An empty SGML element, one with tag(s) but no content (Standard Generalized Markup Language § EMPTY).
An empty line; Optional HTTP message body data; The request/status line and headers must all end with <CR><LF> (that is, a carriage return followed by a line feed). The empty line must consist of only <CR><LF> and no other whitespace. The "optional HTTP message body data" is what this article defines.