This is from an article by Tom Dahm(COO, NetMechanic Inc.)
Of all the changes to IE 6, perhaps the most important for Webmasters is the addition of dual CSS rendering engines. While this new feature helps Internet Explorer comply with the W3C HTML standards, it may also break your existing Web pages.
The new rendering engine changes the rules for writing style sheets so that some things that were permitted in the past are now illegal.

....IE 6 uses the DOCTYPE tag to decide which rendering engine to use. If your page doesn't use a DOCTYPE, then IE 6 will use its classic engine. Also, if you use a DOCTYPE that includes the words "Transitional," then IE 6 will usually use the classic engine. :

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">

However, if your DOCTYPE tag includes the words "Transitional" and gives a URL where the DTD can be found, then IE 6 will instead switch to the new rendering engine. So this version of the tag will use the new engine:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"

Likewise, if your DOCTYPE doesn't include the word "Transitional" or includes the word "Strict," then IE 6 will use the new rendering engine. So either of these DOCTYPE tags will trigger the new rendering engine: