There's a difference between the DTD and the DOCTYPE. The former is a body of rules for the latter. The http://www.w3.org/ website maintains these rules.
With HTML DOCTYPE's less than 5 one must specify the URL of the DTD on the w3.org site, example:
Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
Otherwise IE is in what's known as quirks mode. The menu's generated markup and script code (and many other scripts and markups) do not work well in quirks mode in IE.
All XHTML DOCTYPE's have a URL for the DTD, so if you have a valid XHTML DOCTYPE, you are in standards mode.
Alternatively you may use the very simple HTML 5 DOCTYPE:
which triggers standards mode without a DTD URL in the DOCTYPE.
There is some debate over which is best. Generally though, if you are not serving a true xml page (which if you were, would not work in IE anyway), you should use an HTML DOCTYPE.
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