According to the article below, Frontpage 2000 doesn't
support the doctype tag. I don't use doctype tags when I
use Frontpage (not too often, I'm trying to do stuff
myself) but I suggest you try inserting one and then
trying to validate with the W3C validator and see what
happens. I'd try starting with the HTML 4.01
transitional, I haven't seen a wysiwyg editor yet that
can do xhtml (except maybe Amaya). If you want the first
link is the list of acceptable doctypes, I imagine HTML
3.2 or 4.01 transitional are your best bets. Note that if
you define a doctype, you should validate the page at W3C
(pointless adding one if your page doesn't comply to the
standards).
http://www.w3.org/QA/2002/04/valid-dtd-list.html
http://www.webhelpinghand.com/frontpage2000_things_to_be_a
ware_of.htm