you can include this:
Definition and Usage
The doctype declaration should be the very first thing in an 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 the markup language the page is written in.
The doctype declaration refers to a Document Type Definition (DTD). The DTD specifies the rules for the markup language, so that the browsers render the content correctly.
Doctypes Available in the W3C Recommendations
[b] HTML 4.01 Strict[/b]
[color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
This DTD contains all HTML elements and attributes, but does NOT INCLUDE presentational or deprecated elements (like font). Framesets are not allowed.[/size][/font][/color][color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
[size=3]
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">[/size][/size][/font][/color]
[b] HTML 4.01 Transitional[/b]
[color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
This DTD contains all HTML elements and attributes, INCLUDING presentational and deprecated elements (like font). Framesets are not allowed.[/size][/font][/color][color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
[size=3]
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">[/size][/size][/font][/color]
[b] HTML 4.01 Frameset[/b]
[color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
This DTD is equal to HTML 4.01 Transitional, but allows the use of frameset content.[/size][/font][/color][color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
[size=3]
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">[/size][/size][/font][/color]
[b] XHTML 1.0 Strict[/b]
[color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
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.[/size][/font][/color][color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
[size=3]
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">[/size][/size][/font][/color]
[b] XHTML 1.0 Transitional[/b]
[color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
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.[/size][/font][/color][color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
[size=3]
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">[/size][/size][/font][/color]
[b] XHTML 1.0 Frameset[/b]
[color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
This DTD is equal to XHTML 1.0 Transitional, but allows the use of frameset content.[/size][/font][/color][color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
[size=3]
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">[/size][/size][/font][/color]
[b] XHTML 1.1[/b]
[color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
This DTD is equal to XHTML 1.0 Strict, but allows you to add modules (for example to provide ruby support for East-Asian languages).[/size][/font][/color][color=#000000][font=verdana, helvetica, arial, sans-serif][size=3]
[size=3]
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">[/size][/size][/font][/color]
Source: http://www.w3schools.com/tags/tag_doctype.asp