For all pages using the current HTML standard you should use the following doctype.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
If you are using XHTML instead of HTML then the current doctype is:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
The new versions of HTML and XHTML due to become the standard next year are no longer based on SGML and so don't actually use a doctype but for HTML 5 you still need to include <!DOCTYPE html > to stop the browser going into quirks mode. XHTML doesn't have a quirks mode so there is nothing need be included for XHTML 5.
All doctypes other than the above three are for web pages written using earlier versions of HTML that have been obsolete since 1997.
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