<object>
I did not know you could use the object tag to embed text/html documents. I was researching this and came upon this W3C paper. I doubt this should be your first choice, but the question asked for the possibilities, not recommendations.
http://www.w3.org/TR/WD-frames-970331
IFRAME vs OBJECT
Although a means for creating compound documents is recognized to be a real need the introduction of IFRAME to meet this need is arguable. The main reason is that compound documents can already be created using the existing OBJECT element. Indeed, the purpose of IFRAME is to provide authors with a way to insert HTML documents into an HTML document. At the same time the OBJECT element is designed to provide a general mechanism to insert any type of object into an HTML document. Based on this and given that an HTML document is only a particular type of object, it is reasonable to claim that IFRAME is not necessary, and that OBJECT should be used instead.