Why?
Why specify a doctype? Because it
defines which version of (X)HTML your
document is actually using, and this
is a critical piece of information
needed by some tools processing the
document.
For example, specifying the doctype of
your document allows you to use tools
such as the Markup Validator to check
the syntax of your (X)HTML. Such tools
won't be able to work if they do not
know what kind of document you are
using.
But the most important thing is that
with most families of browsers, a
doctype declaration will make a lot of
guessing unnecessary, and will thus
trigger a "standard" rendering mode.
Source: http://www.w3.org/QA/Tips/Doctype
