We often see <!DOCTYPE ...> as follows:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
What's the functionality of !DOCTYPE?
The most significant use of DOCTYPE is to switch a browser between Quirks Mode and Standards Mode rendering.
This functionality came about because of the "broken" rendering in old versions of IE. It was realised that if Microsoft just "fixed" the IE rendering engine lots of existing sites would not render properly. So the way it works is if you put any valid DOCTYPE declaration at all in your page the assumption is that you know what you're doing and your browser will render in a standards compliant way, and if you don't put one in it will render in the old "wrong" way.
This was originally done in IE for the Mac, but this behaviour is the same in all versions of IE since IE5, and Firefox, Chrome, Safari and Opera.
What the DOCTYPE is supposed to be is a Document Type Definition. HTML is subset of SGML (as is XML). The DTD tells a parser which syntax you are using. So in a webpage your DOCTYPE should match the version of HTML you are using.
It tells to the validator which version of HTML do are you using. A browser use this information to render the page correctly.
Here are correct version of DOCTYPE:
HTML 4.01 Strict, Transitional, Frameset
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
XHTML 1.0 Strict, Transitional, Frameset
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
XHTML 1.1
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
HTML5
<!DOCTYPE html>
Mainly for compatibility.
Modern browsers don't need it anymore. See this HTML file that does not have a !DOCTYPE:
<p>hello</p>
The browser automatically inserts the !DOCTYPE.
But that's only in modern browsers.
In IE8, some css properties only have functionality when the doctype is stated.
So, if you don't need to support ancient relic browsers, you don't need it. But it has become a programmer custom!
———
Update:
The doctype used to require a DTD URL. The DTD defines all the elements in the document. If you are using the HTML 4.01 DTD, HTML 5 elements will not work.
Some features will break. Without a DOCTYPE, the green square fills up 50% of the width:
div {
width:50%;
background-color:#00ff00;
}<div>div</div>But with one:
div {
width:50%;
background-color:#00ff00;
}<!DOCTYPE html>
<div></div>However, stack snippets automatically adds a !DOCTYPE, so you can't see it here.
It tells your browser which version of HTML it is loading making the load time a bit quicker as the browser know what to expect.
The doctype is a standard defined by the w3c - when you hear about standards based web development this is what they are talking about. The idea of using the doctype is you create valid HTML that follows the doctype.
If you are clever you can actually write your own doctype.
The main (practical) purpose of DOCTYPEs is to force IE from "quirks mode" to "standards-compliant mode", both euphemisms for "horribly broken mode" and "slightly broken mode" (respectively).
In other browsers, the page responds normally without the !DOCTYPE, with these exceptions:
z-index CSS property in Internet Explorer. In quirks mode, it does not work. You need to be in standards-compliant mode for it to work. The easiest way to do this, of course, is with a DOCTYPE. <div>some text</div>
<style type="text/css">
div {
background-color:#00ffff;
height:10%;
width:10%;
}
</style>
Without a !DOCTYPE, the element is sized according to CSS. But with the !DOCTYPE, it is sized according to the element.The !doctype was not that simple back then. You had to specify a DTD URL:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
The DTD looks something like this:
<!ENTITY % HTML.Version "-//W3C//DTD HTML YOURVERSION Transitional//EN"
-- this is a comment.
-->
<!ENTITY % HTML.Frameset "IGNORE">
<!ENTITY % ContentType "CDATA"
-- media type, as per [RFC2045]
-->
<!ENTITY % ContentTypes "CDATA"
-- comma-separated list of media types, as per [RFC2045]
-->
<!ENTITY % Charset "CDATA"
-- a character encoding, as per [RFC2045]
-->
<!ENTITY % Charsets "CDATA"
-- a space-separated list of character encodings, as per [RFC2045]
-->
<!ENTITY % LanguageCode "NAME"
-- a language code, as per [RFC1766]
-->
<!ENTITY % Character "CDATA"
-- a single character from [ISO10646]
-->
<!ENTITY % LinkTypes "CDATA"
-- space-separated list of link types
-->
<!ENTITY % MediaDesc "CDATA"
-- single or comma-separated list of media descriptors
-->
<!ENTITY % URI "CDATA"
-- a Uniform Resource Identifier,
see [URI]
-->
<!ENTITY % Datetime "CDATA" -- date and time information. ISO date format -->
<!ENTITY % Script "CDATA" -- script expression -->
and so on an so forth.
