HyperText Markup Language

HTML Prologue

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html40/loose.dtd">

The prologue appears at the beginning of every HTML page, identifies what follows as an HTML document allowing browsers and other special software to distinguish HTML documents from other types (DTDs) of SGML. All HTML documents written according to the current HTML specification (Version 4.01) should use the prologue tag displayed above.

            1     2     3   4    5         6                 7
 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" >

HTML - The SGML document type being declared: <HTML> ... </HTML>
PUBLIC - Identifies the information in quotes as a Formal PUBLIC Identifier.
"-" - The minus sign designates unregistered organization. ISO, registered (+) or unregistered (-) are possibles here. W3C is not currently registered with ISO, therefore the (-) is used.
W3C - identifies the party responsible for creation/maintenance of the DTD. If the DTD comes from IETF, W3C, etc. you'll see their ID here.
DTD - describes the type of object, called a Public Text Class. In this case, it is a DTD.
HTML 4.01 Transitional - is the Public Text Description. Here you'll find the DTD's name, plus flavors such as version numbers, "strict", "draft," "transitional," etc.
EN - identifies the Public Text Language, describing the natural language in which the public text is written, represented by two, uppercase-only characters from ISO 639. "EN"= English.

Flavors of HTML 4.01

HTML 4.01 is specified in three "flavors". You specify which of these variants you are using by inserting a line at the beginning of the document. For example, the HTML for this document starts with a line which says that is it using HTML 4.01 Transitional. Thus, if you want to validate the document, the tool used knows which variant you are using. Each variant has its own DTD - Document Type Definition - which sets out the rules and regulations for using HTML in a succinct and definitive manner

HTML 4.01 Transitional - Most people writing Web pages for the general public to access will want to use this flavor of HTML 4.01. The idea is to take advantage of HTML 4.01 features including style sheets but nonetheless to make small adjustments to your mark-up for the benefit of those viewing your pages with older browsers which can't understand style sheets. These include using BODY with bgcolor, text and link attributes.
HTML 4.01 Strict - Use this when you want really clean structural mark-up, free of any tags associated with layout. Use this together with W3C's Cascading Style Sheet language (CSS) to get the font, color, and layout effects you want.
HTML 4.0 Frameset - Use this when you want to use HTML Frames to partition the browser window into two or more frames.

HTML 4.01 specifies three DTDs, so authors must include one of the following document type declarations in their documents. The DTDs vary in the elements they support.

The HTML 4.01 Transitional DTD includes everything in the strict DTD plus deprecated elements and attributes (most of which concern visual presentation). For documents that use this DTD, use this document type declaration:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html40/loose.dtd">

The HTML 4.01 Strict DTD includes all elements and attributes that have not been deprecated or do not appear in frameset documents. For documents that use this DTD, use this document type declaration:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html40/strict.dtd">

The HTML 4.01 Frameset DTD includes everything in the transitional DTD plus frames as well. For documents that use this DTD, use this document type declaration:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html40/frameset.dtd">

The Library of Congress will use the HTML 4.01 Transitional DTD in its initial HTML implementation.

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Library of Congress Help Desk (October 16, 2000)