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How To Declare DOCTYPE In HTML Documents?

The <!DOCTYPE> declaration must be the very first thing in the HTML document, before the <html> tag.

The <!DOCTYPE> declaration is not an HTML tag; it is an instruction to the web browser about what version of HTML the page is written in.

In the previous versions of HTML, the <!DOCTYPE> declaration refers to a DTD (Document Type Definition), because they are based on SGML (Standard Generalized Markup Language).

The DTD specifies the rules for the markup language, so that the browsers render the content correctly. HTML5 is not based on SGML, and so does not require a reference to a DTD.

The <!DOCTYPE> declaration is NOT case sensitive.

Common <DOCTYPE> Declarations:
HTML 5:
<!DOCTYPE html>

HTML 4.01 Strict: This DTD contains all HTML elements and attributes, but does not include deprecated elements (like font). Framesets are not allowed.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">

HTML 4.01 Transitional: This DTD contains all HTML elements and attributes, including presentational and deprecated elements (like font). Framesets are not allowed.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

HTML 4.01 Frameset: This DTD is equal to HTML 4.01 Transitional, but allows the use of frameset content.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">

XHTML 1.0 Strict: This DTD contains all HTML elements and attributes, but does not include presentational or deprecated elements (like font). Framesets are not allowed. The markup must also be written as well-formed XML. 

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

XHTML 1.0 Transitional: This DTD contains all HTML elements and attributes, including presentational and deprecated elements (like font). Framesets are not allowed. The markup must also be written as well-formed XML.

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

XHTML 1.0 Frameset: This DTD is equal to XHTML 1.0 Transitional, but allows the use of frameset content.

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

XHTML 1.1: This DTD is equal to XHTML 1.0 Strict, but allows to add modules. 

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

A very simple HTML5 web page can be as simple as below: 

<!DOCTYPE html>
<html>
<head>
<title>HTML5 is rocks!</title> </head>
<body>
<h1>HTML5 Rocks!</h1> </body>
</html>

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