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Other document markup languages are partly related to SGML and XML, but—because they cannot be parsed or validated or otherwise processed using standard SGML and XML tools—they are not considered either SGML or XML languages; the Z Format markup language for typesetting and documentation is an example.
An SGML document is a text document with SGML markup defined in an SGML prologue (i.e., the DTD and subdocuments). A complete SGML document comprises not only the document instance itself, but also the prologue and, optionally, the SGML declaration (which defines the document's markup syntax and declares the character encoding). [1]
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.
XML (Extensible Markup Language) is a meta markup language that is very widely used. XML was developed by the World Wide Web Consortium in a committee created and chaired by Jon Bosak. The main purpose of XML was to simplify SGML by focusing on a particular problem — documents on the Internet. [23]
A processing instruction (PI) is an SGML and XML node type, which may occur anywhere in a document, intended to carry instructions to the application. [1] [2]Processing instructions are exposed in the Document Object Model as Node.PROCESSING_INSTRUCTION_NODE, and they can be used in XPath and XQuery with the 'processing-instruction()' command.
Any valid SGML or XML document that references an external subset in its DTD, or whose body contains references to parsed external entities declared in its DTD (including those declared within its internal subset), may only be partially parsed but cannot be fully validated by validating SGML or XML parsers in their standalone mode (this means ...
The earliest transformation languages predate the advent of XML as an SGML profile, and thus accept input in arbitrary SGML rather than specifically XML. These include the SGML-to-SGML link process definition (LPD) format defined as part of the SGML standard itself; in SGML (but not XML), the LPD file can be referenced from the document itself by a LINKTYPE declaration, similarly to the ...
With the exception of the lack of a URI or the FPI string (the FPI string is treated case sensitively by validators), this format (a case-insensitive match of the string !DOCTYPE HTML) is the same as found in the syntax of the SGML based HTML 4.01 DOCTYPE. Both in HTML4 and in HTML5, the formal syntax is defined in upper case letters, even if ...