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Jun 24, 2016 am 11:45 AM

DOCTYPE is the abbreviation of Document type, indicating what version of XHTML or HTML is used. Must appear before the tag and does not need to close the tag.

The declaration is not a tag, it instructs the web browser as to which version of HTML the page was written in. Always add the declaration to your HTML document so that the browser knows the document type. How to ultimately display your web document. Therefore, if you want to create a page that complies with W3C standards, the DOCTYPE statement is an essential and key component.

Styling issues caused by lack of HTML Doctype

When a web page is developed using the XHTML standard, the first line is:

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

This is xhtml W3C standard.

DOCTYPE specifies the document type definition (DTD) that the HTML document follows. The declaration follows certain rules and indicates what set of rules the reading program uses to interpret the tags in the document.

Rule style:

Syntax:

　 HTML Top-level element availability "registration // organization // type // tag // definition // language" "URL"

Possible values:

Top-level element: The top-level element type declared in the specified DTD, which corresponds to the declared SGML document type. Default is: HTML

Availability: Specifies whether the Formal Public Identifier (FPI) is an accessible object or a system resource. The default is: PUBLIC, a publicly accessible object. SYSTEM System resources, such as local files or URLs.

Registration: Specify whether the organization is an International Standards Organization (ISO). default. The organization name has been registered.

Organization name is not registered: IETF and W3C are not registered ISO organizations.

Organization: Specify who is responsible! The DOCTYPE declaration refers to the name of the team or organization that created and maintains the DTD.

Type: Specifies the public text class, that is, the type of object being referenced. Default: DTD.

tag: Specifies a public text description, a unique descriptive name for the referenced public text. Can be accompanied by a version number. Default: HTML

Definition: Specifies the document type definition. Frameset frameset documentation. Strict excludes all representative attributes and elements that W3C experts want to phase out. Transition contains all contents excluding Frameset elements.

Language: Specify the public text language, the natural language encoding system that creates the referenced object. Default: EN.

URL: Specifies the location of the referenced object.

(reprinted from: http://www.cnblogs.com/tomatoxml/p/3585741.html)

　DTD (in the example: xhtml1-transitional.dtd) is the document type definition, Containing the layout of the document, the browser interprets the markup of the page according to the defined DTD and then displays it.

xhtml1.0 provides three DTD declarations to choose from:

Transitional: Contains all HTML elements and attributes, including presentational and deprecated elements, and does not allow the use of framesets (Framesets) ,XML must be written in the correct format.

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

Strict: Contains all HTML elements and attributes, but does not include presentational and deprecated elements (such as: font). Framesets are not allowed and must be Write XML in the correct format.

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

Frameset: Equivalent to XHTML 1.0 Transitional, but allows frameset content.

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

Compared with the three DTD declarations of xhtml and the three declarations of HTML 4.01, there is only one declaration in HTML5:

<!DOCTYPE html>

In HTML4.01, the statement refers to DTD. Because HTML4.01 is based on SGML, DTD specifies the planning of the markup language so that the browser can display it correctly. Content, HTML5 is not based on SGML, so there is no need to reference the DTD.

Explain about XHTML1.0, HTML 4.0 and HMTL5:

XHTML 1.0 is a transformation of HTML 4.0 in accordance with the XML standard requirements. It is an XML-styled HTML4.01, while HTML5 is A generation of HTML to replace HTML 4.01. The W3C did originally plan to replace HTML 4.01 with the (can be born), it died before it could escape the shadow of HTML 4.01. Before the rise of XHTML, its status was replaced by HTML5. (Excerpted from http://www.zhihu.com/question/20258628)

Following the DOCTYPE declaration is an XHTML namespace:

<html xmlns="http://www.w3.org/1999/xhtml">

When you need to use an XHTML document that conforms to the XML specification, you should use the xmlns attribute in the tag.

Check Tool: If you need to check whether your page complies with the standards declared in DOCTYPE, you can use the verification tool provided by W3C: http://validator.w3 .org/

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