Technical Interview Questions & Answers: DTD

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DTD's Example

The first line of many World Wide Web pages reads as follows:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

This Document Type Declaration for XHTML includes by reference a DTD, whose public identifier is -//W3C//DTD XHTML 1.0 Transitional//EN and whose system identifier is http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd. An entity resolver may use either identifier for locating the referenced external entity. The root element is declared to be html.

Syntax DTD's

The general syntax for a Document Type Declaration is:

<!DOCTYPE root-element [SYSTEM OR PUBLIC FPI] "uri" [ 
<!-- internal declarations -->
]>

In XML, the root element of the document is the first element in the document (In XHTML, the root element is html - being the first element opened and last closed). The keywords SYSTEM and PUBLIC suggest what kind of DTD it is (one that is on a private system or one that is open to the public). If the "SYSTEM" keyword is chosen, the URI of the DTD may follow; if the PUBLIC keyword is chosen then the declaration must have a FPI (Formal Public Identifier) following it and then the URI of the DTD (the FPI for XHTML 1.1 is "-//W3C//DTD XHTML 1.1//EN").

The last part is called an internal subset which can be used to add/edit entities or add/edit elements. This is optional in XML.

Document Type Declarations are slightly different in SGML, where you may associate the public identifier with the system identifier, as it is in HTML.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">

Syntax XHTML Mobile Profile DTDs

XHTML Mobile Profile 1.0

<!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.0//EN" 
"http://www.wapforum.org/DTD/xhtml-mobile10.dtd">

XHTML Mobile Profile 1.1

<!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.1//EN" 
"http://www.openmobilealliance.org/tech/DTD/xhtml-mobile11.dtd">

XHTML Mobile Profile 1.2

<!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.2//EN" 
"http://www.openmobilealliance.org/tech/DTD/xhtml-mobile12.dtd">

Syntax XHTML Basic 1.0 and 1.1 DTDs

XHTML Basic 1.0

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
 "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">

XHTML Basic 1.1

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.1//EN"
 "http://www.w3.org/TR/xhtml-basic/xhtml-basic11.dtd">

What is XHTML 1.1 DTD?

This is the latest DTD that has the stringency of XHTML 1.0 Strict, and it is based upon the module framework and modules defined in Modularization of XHTML.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

What is XHTML 1.0 DTDs?

XHTML's DTDs are also Strict, Transitional and Frameset.

XHTML Strict DTD is the most strict DTD available: no deprecated tags are supported and the code must be written correctly.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

XHTML Transitional DTD is like the XHTML Strict DTD, but deprecated tags are allowed. This is the most popular current DTD.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

XHTML Frameset DTD is the only XHTML DTD that supports Frameset. The DTD is below.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">

what is HTML 5?

HTML 5 has a unique doctype, which is very short and has no Document Type Definition. This is how it appears:

<!DOCTYPE HTML>

What is Document Type Declaration or DOCTYPE?

A Document Type Declaration, or DOCTYPE, is an instruction that associates a particular SGML or XML document (for example, a webpage) with a Document Type Definition (DTD) (for example, the formal definition of a particular version of HTML). In the serialized form of the document, it manifests as a short string of markup that conforms to a particular syntax.

Despite its name, a Document Type Declaration is not suitable for deducing the type of the document, although apparently it was originally supposed to be.

The HTML layout engines in modern web browsers perform DOCTYPE "sniffing" or "switching", wherein the DOCTYPE in a document served as text/html determines a layout mode, such as "quirks mode" or "standards mode". The text/html serialization of HTML 5, which is not SGML-based, uses DOCTYPE only for mode selection.

What is Document Type Definition (DTD) ?

Document Type Definition (DTD) is one of several SGML and XML schema languages, and is also the term used to describe a document or portion thereof that is authored in the DTD language. A DTD is primarily used for the expression of a schema via a set of declarations that conform to a particular markup syntax and that describe a class, or type, of document, in terms of constraints on the structure of that document. A DTD may also declare constructs that are not always required to establish document structure, but that may affect the interpretation of some documents. XML documents are described using a subset of DTD which imposes a number of restrictions on the document's structure, as required per the XML standard (XML is in itself an application of SGML optimized for automated parsing).

DTD is native to the SGML and XML specifications, and since its introduction other specification languages such as XML Schema and RELAX NG have been released with additional functionality.

As an expression of a schema, a DTD specifies, in effect, the syntax of an "application" of SGML or XML, such as the derivative language HTML or XHTML. This syntax is usually a less general form of the syntax of SGML or XML.