Q:

Using  tag to include HTML content: allowed format of included file

OK, I know how to include HTML content from a separate file using the <object> tag. What I can't find any info about is what is allowed/required within the included HTML file. Can said included file merely be some text with some HTML tags, or does it have to be a complete HTML file with headers, <head>, and <body>? How does this appear within the DOM of the original document, if it appears within that DOM at all? Or are the two documents treated entirely separately?
Yes, I know, I could experiment to see what works. However, I know enough about HTML to know that what happens to work, for now, may not be the correct way to do things. I am not expecting anyone to list out all the rules here, but if someone could post some links I would much appreciate it. This is a topic that has proven exceeding difficult to search the internet for.

A:

In 13.5 Notes on embedded documents, I believe I have found the answer to both of my questions. The second paragraph says,

An embedded document is entirely independent of the document in which
  it is embedded. For instance, relative URIs within the embedded
  document resolve according to the base URI of the embedded document,
  not that of the main document. An embedded document is only rendered
  within another document (e.g., in a subwindow); it remains otherwise
  independent.

So, yes, as both @Quentin and @Sinan said, it would require the embedded .html file to be a complete, valid .html file. And, no, it would not become part of the DOM of the original document.
Thanks to everyone for their prompt assistance. The StackOverflow community continues to amaze me. 

