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A document type definition (DTD) is a specification file that contains set of markup declarations that define a document type for an SGML-family markup language (GML, SGML, XML, HTML). The DTD specification file can be used to validate documents. A DTD defines the valid building blocks of an XML document.
A document type declaration, or DOCTYPE, is an instruction that associates a particular XML or SGML document (for example, a web page) with a document type definition (DTD) (for example, the formal definition of a particular version of HTML 2.0 - 4.0). [1]
This declaration references the DTD for the "strict" version of HTML 4.01. SGML-based validators read the DTD in order to properly parse the document and to perform validation. In modern browsers, a valid doctype activates standards mode as opposed to quirks mode. In addition, HTML 4.01 provides Transitional and Frameset DTDs, as explained ...
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.
As HTML (before HTML5) is based on SGML, [2] its parsing also depends on the Document Type Definition (DTD), specifically an HTML DTD (e.g. HTML 4.01 [3] [note 1]). The DTD specifies which element types are possible (i.e. it defines the set of element types) and also the valid combinations in which they may appear in a document.
XML does not use the grammar (DTD) to change delimiter maps or to inform the parse modes, and does not allow tag omission; consequently, XML validation of elements is not active in the sense that SGML validation is active. SGML without a DTD (e.g. simple XML), is a grammar or a language; SGML with a DTD is a metalanguage.
An FPI consists of an owner identifier, followed by a double slash (//), followed by a text identifier. [1]: 381–382 For example, the identifier "-//W3C//DTD HTML 4.01//EN" can be broken down into two parts: the owner identifier which indicates the issuer of the FPI, and the text identifier which indicates the particular document or object the FPI identifies. [2]
HTML that has been written to conform to both the HTML and XHTML specifications and therefore produces the same DOM tree whether parsed as HTML or XML is known as polyglot markup. [124] There is no DTD for XHTML5. [125]