Definition of doctype Tag
Document type declaration. This is used to let the browser know what version of HTML you are using. If you don't use it, or if you get it wrong, the browser will assume you don't know what you're doing and switch to 'quirks mode', which will not render things as they should be. Apparently it's more 'forgiving' but it actually seems to be quite random and confusing.
The case must be like that in the example below (with upper case 'DOCTYPE'). It does not close like other tags.
Code Example with doctype Tag
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">