It always sets alarm bells ringing for me when users want to display html content in iframes or object tags as I have difficulty in understanding the logic. Why not display the content in the page to start with in the normal way and get and get a regular indexable and accessible page?
If the content is important then it should be in the page to start with.
I realise that sometimes we may be working with already created pages but wouldn’t it be better to put those pages in your site to start with (assuming they are under your control).
Sometimes users use iframes so that they can swap the content while staying on the same page but that is just mimicking html Frames and no one uses frames these days due to their accessibility issues. That’s one of the reasons that iframes were deprecated in xhtml but brought back in html5 to cater for things like videos etc where it make sense for them to be frames.
Html doesn’t need to be framed and indeed you could fetch the snippets of html with js if you wanted live updates (ajax) but then again you don’t really have an indexable or accessible site if all you do is swap content in and out of one page.
It may indeed be that your use case is indeed valid for iframe usage which is fine but generally I find that it is developers being slightly lazy and not doing it correctly from the start.
Regarding xhtml then yes it is stricter than html5 and for that reason ‘old school coders’ like it better than html5 but the fact that it is better for older browsers is nonsense. Html5 will work just as well as long as you don’t use any of the new tags. However if I asked you to use your television without using electricity then you would say I was stupid.
You can’t support everything that went before and indeed if you do encourage people with older browsers to not update then they risk their security as all the ransomeware attacks of the last few years have been targeted against old and vulnerable browsers. We need to let any browser that is not supported by its current vendor to fade away.
Note that xhtml is generally fake xhtml anyway but that’s an argument for another day ;).