That page is just an informal summary. It is very useful, but in a
dispute or uncertainty, the formal definitions and normative prose
should be checked.
The DTD column there is partly misleading, since some (actually,
most) elements that have L there should have both L and F. Moreover,
the empty cells should be read "this element appears in all DTDs",
which is not immediately obvious from the page.
The Frameset DTD is just a variant of the Transitional DTD, as you
can see from
http://www.w3.org/TR/REC-html40/sgml/framesetdtd.html
which really says that the DTDs are identical except for
<!ENTITY % HTML.Frameset "INCLUDE">
which makes some parts of the Transitional DTD enabled in Frameset
variant (only).
In particular, the FRAMESET element may contain the NOFRAMES part
(commonly used to demonstrate the author's cluelessness, such "This
page requires frames"), which in turn contains a BODY element, which
may contain anything that the BODY of a Transitional document may
contain. This of course includes IFRAME.
On the other hand, it would be "somewhat" weird to have IFRAME inside
an element whose content is ignored on any frames-enabled browser.
(It is remotely possible that some odd browser somewhere has a mode
where "normal" frames are disabled but inline frames are enabled. I
never saw one... but actually I think one could easily turn a normal
browser into such an oddity, by using simple CSS that sets frame {
display: none; }. But why?)
