That shouldn't even be a thing that's allowed just from a UX and accessibility standpoint. And that's also when I'd fake the appearance of that with modal dialogs that trigger each-other. Maybe top-align the inner DIV and who knows the diff?

Would let you be more versatile in the content without breaking the accessibility of the semantics too. From a presentation standpoint the client might not even know the difference, even though you end up not nesting them.

--edit-- in fact such an approach might make for a decent article I could write. Thanks for being my muse. :D