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.
<--!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 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">
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".)
