[Off-topic] HTML 4.01 and the IFRAME element

Not a validator question (although it is the validator
that has drawn it to my attention) but is it really the
case that in HTML 4.01, one cannot use the IFRAME element
except in conjunction with the HTML 4.01 FRAMESET DTD ?
If so, that does seem a very odd restriction, and not
one that I really understand.

Philip Taylor