DOCTYPE (short for Document Type Declaration) is something that you often see it (via HTML page source), and perhaps work with it all the time (if you are working on the web). DOCTYPE element usually appears on top of a well formed HTML/XHTML page. It looks like
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
or
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
What does it do?
DOCTYPE tells the rendering browser the markup specification that the page adheres to. Let's analyze one of the sample DOCTYPE:
<!DOCTYPE html
This is HTML markup
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
The markup follows "XHTML 1.0 Transitional" standard
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
The markup on this document follows
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" XHTML DTD convention.
Why do we need it?
When a browser renders a web page, it wants to know what type of markup the page conforms to. Knowing the markup format, the browser can make sure that the CSS stylesheet and style attributes in the markup are rendered properly. Web designers can be sure that the markup output will behave as he intends it to.
What if I don't have one?
Most popular browsers (IE, FireFox, Safari, Netscape) can operate in quirk mode when DOCTYPE markup is not presented or malformed. In quirk mode the browser will have to guess what type of HTML mark up the page adheres to, whether it's HTML 3.2, 4.0 or one of the XHTML markup types. You really should specify DOCTYPE to ensure consistency of rendered output.
Well, if it looks good on my deploying browsers' quirk mode, why else should I care?
If you are running the browser on a powerful PC, it should be no problem. Yes, no problem if you are willing to segment out billions who run browser on mobile and embedded devices. All mobile platform developers know that on these kind of devices, every byte of memory matters and tight control over browser rendering engine is essential . There are also incredible number of machines running IE 5 and Netscape 4.x under 256MB of RAM, in that environment.
Sadly, I didn't know or care much about what DOCTYPE does until recently. It used to be my 'copy-n-paste' kinda thing =) and it still is for many others.
