The DOCTYPE declaration should be the very first thing on the page. It tells the browser what kind of HTML you're using. A lot of packaged systems omit it.

Leaving it out may not make much difference to the way most users see your site, since browsers generally guess, but it makes it next to impossible to debug your code, since debugging tools will spit out a vast number of error messages, most of which amount to "I can't be sure what this means" - so you can't see the real problems in the flood.