A DOCTYPE Definition is a special declaration at the very top of your Webpage source – right above the HTML tag – that informs validators the rules in which to validate your page using, and for modern browsers (IE6+, Firefox, NS6+, Opera, IE5 Mac), whether to display your page in Quirks or Standards mode.

Below lists the major doctypes you can deploy on your webpage. All of them enters modern browsers into “Standards” mode when used.

HTML 4.01 Transitional, Strict, Frameset

A HTML 4.01 Transitional DOCTYPE supports all the attributes of HTML 4.01, presentational attributes, deprecated elements, and link targets. It is meant to be used for webpages that are transitioning to HTML 4.01 strict.

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

HTML 4.01 Strict is a trimmed down version of HTML 4.01 with emphasis on structure over presentation. Deprecated elements and attributes (including most presentational attributes), frames, and link targets are not allowed. CSS should be used to style all elements.

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

HTML 4.01 Frameset is identical to Transitional above, except for the use of FRAMESET over BODY.

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

XHTML 1.0 Transitional, Strict, Frameset

Use XHTML 1.0 Transitional when your webpage conforms to basic XHTML rules, but still uses some HTML presentational tags for the sake of viewers that don’t support CSS.

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

Use XHTML 1.0 Strict when your webpage conforms to XHTML rules and uses CSS for full separation between content and presentation.

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

XHTML 1.0 Frameset is identical to Transitional above, except in the use of the FRAMESET tag over BODY:

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

XHTML 1.1 DTD

XHTML 1.1 declaration. Visit the WC3 site for an overview and what’s changed from 1.0.

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