Thank you so much, that is indeed valuable, and shows that it is possible.
I wonder if the reason could be that I made the input file with LibreOffice, so perhaps it is not a "completely authentic" DOCX...
Anyhow, I suppose one way to fix the issue for my book would be to open the demo file after EPUB conversion in the calibre editor and see what code is used to gnerate this effect, and then simply copy it and simply paste it into my project....
Thank you so much!