So, to get the expected behavior (like that of a W3C compliant browser) you have to add a specific Qt CSS property ?! (or to be exact, kept)
...
Whoah.
Great.
Absolutely obvious.
*sighs*
All right, then. Just in case, could you try <p></p> if it works?
I know currently than Calibre filter and delete empty paragraphs, but if it works, it would be probaly a more elegant solution.