Doctype

!Doctype End Tag: NA Support Key: 2 \| 3 \| 3.2 \| 4 \| IE \| M \| N \| O	What is it? Attributes Tag Example	Parent/Content Model Tips & Tricks Browser Peculiarities
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What is it?: The !Doctype declaration is a top-level element known as a Public Text Identifier. It should appear at the very beginning of an HTML document in order to identify the content of the document as conforming (theoretically) to a particular HTML DTD specification. Almost all popular browser parsers allow the omission of this declaration (probably because they are expecting HTML and they interpret only those elements and attributes which they can handle.) The quoted segment within a DOCTYPE declaration is called a Formal Public Identifier (FPI.) Every distinct DTD variation will have its own unique FPI string.

Syntax

Typical HTML DOCTYPE statement:

<!DOCTYPE	HTML	PUBLIC	"-//	IETF//	DTD	HTML//	EN">
	⁽¹⁾	⁽²⁾	⁽³⁾	⁽⁴⁾	⁽⁵⁾	⁽⁶⁾	⁽⁷⁾

Annotated syntax:

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

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 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 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 DTD, it is "DTD" - a Document Type Definition.

(6) : [Label] - This is the "Public Text Description" - a unique descriptive name for the public text being referenced. If the public text changes for any reason, a new Public Text Description 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 DTDs are usually (always?) written in English ("EN".)

[Note: If you are using Netscape and most of the above content is invisible, you are seeing an interesting and elusive Netscape rendering bug. A fix that works for me is to expand the browser window (if window is not already maximized and resolution permits) at least wide enough for the "Annotated Syntax" line to be visible on a single line. I have been trying to track down the reason for this behavior for a long time but it has been exceedingly hard to reproduce... this fix works on MY machine at least. Weird, no?]

Attributes: This element does not accept any attributes.

!DOCTYPE Examples
HTML 2.0:   <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0 Level 2//EN">
  <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
  <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
  <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML Level 2//EN">
HTML 3.0: <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 3.0//EN//">
HTML 3.2: <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
HTML 4.0:   <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Frameset//EN">

Parent Model: NA
Content Model: <html>

Tips & Tricks

Some HTML validation tools will need to have the !DOCTYPE statement present in order to choose the proper DTD to validate against.
Using a DOCTYPE statement allows SGML tools (such as SGML browsers or document validators) to view your document. Some of these tools can play vital roles in managing HTML documents.
SGML tools can also use URLs as FPI references to DTDs. The SYSTEM keyword in the Public Text Identifier indicates that the FPI may be a URL reference. Example:
<!DOCTYPE HTML SYSTEM "http://www.w3.org/DTD/HTML4-strict.dtd">

Browser Peculiarities

No popular browsers seem to care about this declaration (because they expect the content of the documents to be HTML.)

Related Sites

Official References
http://info.internet.isi.edu:80/in-notes/rfc/files/rfc1866.txt: RFC 1866: The HTML 2.0 specification (plain text)
http://www.w3.org/MarkUp/html-spec: The web version of the HTML 2.0 (RFC 1866) specification
http://www.w3.org/MarkUp/Wilbur/: The HTML 3.2 (Wilbur) recommendation
http://www.w3.org/TR/REC-html40/cover.html: The HTML 4.0 Specification Draft

!Doctype