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本文详细解析了HTML文档类型声明（DOCTYPE）的各个组成部分及其意义，包括顶级元素、可用性、注册标识、组织名称、类型、标签及语言等，并提供了典型的HTML DOCTYPE声明实例。

Typical HTML DOCTYPE statement:

<!DOCTYPE	HTML	PUBLIC	"- //	W3C //	DTD	HTML 4.0 Transitional //	EN "	"http://www.w3.org/TR/html4/loose.dtd ">
	⁽¹⁾	⁽²⁾	⁽³⁾	⁽⁴⁾	⁽⁵⁾	⁽⁶⁾	⁽⁷⁾	⁽⁸⁾

Annotated syntax:

<!DOCTYPE	[Top Element ]	[Availability ]	"[Registration ]//	[Organization ]//	[Type ]	[Label ]//	[Language ]"	"[URL ]">
	⁽¹⁾	⁽²⁾	⁽³⁾	⁽⁴⁾	⁽⁵⁾	⁽⁶⁾	⁽⁷⁾	⁽⁸⁾

Explanation

General : A pair of forward slash characters ("//") is used as delimiters between keyword fields in the FPI declaration.

(1) : [Top Element ] - Indicates the top level element type declared in the DTD; for HTML it is <html >.

(2) : [Availability ] - field indicates whether the identifier is a publicly accessible object (PUBLIC) or a system resource (SYSTEM) such as a local file or URL. HTML/XHTML DTDs are specified by PUBLIC identifiers.

(3) : [Registration ] - Indicated by either a plus ("+") or minus ("-"). A plus symbol indicates that the organization name that follows is ISO-registered. A minus sign indicates the organization name is not registered. The IETF and W3C are not registered ISO organizations and thus use a "-".

(4) : [Organization ] - This is the "OwnerID" - a unique label indicating the name of the entity or organization responsible for the creation and/or maintenance of the artifact (DTD, etc.) being referenced by the DOCTYPE. The IETF and W3C are the two originating organizations of the official HTML/XHTML DTDs.

(5) : [Type ] - This is the "Public Text Class" - the type of object being referenced. There are many different keywords possible here, but in the case of an HTML/XHTML DTD, it is "DTD" - a Document Type Definition.

(6) : [Label ] - This is the "Public Text Description" - a unique descriptive name for the public text (DTD) being referenced. If the public text changes for any reason, a new Public Text Description string should be created for it.

(7) : [Language ] - This is the "Public Text Language"; the natural language encoding system used in the creation of the referenced object. It is written as an ISO 639 language code (uppercase, two letters.) HTML/XHTML DTDs are usually (always?) written in English ("EN".)

(8) : [URL ] - This is the optional explicit URL to the DTD being referenced.