XHTML page causes horizontal scrolling when inside a frame?

I have an XHTML page that, for all intent and purpose, is the exact same
page I had embedded in my frame as HTML 4.0 transitional, except now I have
changed the header to the "transitional" XHTML DTD at the top and have
changed my tags to ensure they are valid or "well-formed."

Now the page appears with a horizontal scroll bar when embedded inside a
frame, even though the content all fits on the screen.  If I remove the DTD
header from the top, then the horizontal scroll bar disappears.

What is it about the transitional XHTML DTD that causes the horizontal
scroll bar?  Is it a known issue?  I also made sure the frameset itself was
using the XHTML frameset DTD, and had the same results.   Also, the frameset
is using the usual scrolling attributes to determine if a page should scroll
or not based on how it fits in the window.  Clearly, it's not working
correctly in XHTML.  The scrollbar does not appear if you open the page by
itself with the DTD --it only happens when it's part of a frameset.

Thanks,
Mike

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