> iframe is a full page.

An iframe loads whatever you give it. If you give it a URL to a "full page"/"whole page", then it loads that. If the URL leads to e.g. "a div" or "a table", then that will be loaded. That's under the control of the person who's putting stuff on the other end of the URL. To repeat, there is exactly one case where this doesn't hold up: when you want the loaded content to be inline and not block-level (e.g. "some text"—and if inline content is what you meant, then you should say that instead of just giving another vague response; again, though, even if that is what you want, how would that be relevant to the use case we're talking about here: embedding tweets?)