Beware, I am just a beginner on CSS. This time I succeeded to reduce the white space between paragraphs.
The epub text is coming from a foreign source and has been converted from an odt file by calibre as an EPUB .
I opened this epub file with Sigil 0.2, I looked at an xhtml file with code view and I identified the P11 paragraph style as being the most used (why? mystery?) : see joint
texte.png on the right
I then opened the css stylesheet at P11 and reduced to zero the values of margin-bottom and margin-top which were set at 1em (all the others values related to margin and padding were already at zero): see joint
style.png
That was enough to do the trick.
I am not sure at all this is academic but it works. Now I have one line between paragraphs instead of three.