Sorry, I should have included in my post - ePub. First, I must disclose that I am a complete, untrained noob with HTML and CSS. That said, I have learned to work with paragraphs, margins, indents, spacing, etc. by studying the stylesheets along with excellent information from those here on MR and tutorials on
www.ws3.org.
The ePubs display fine when viewing in Calibre's viewer and Sigil but introduce additional line spacing when side-loading to a Kobo (Mini). So, I removed the unwanted spacing by converting ePub => ePub along with the method described in my original post. That's where the unwanted removal of spacing in other areas appeared resulting in my attempt to fix it. I copied what was in the original ePub to restore spacing. For example:
<p class="x2DED-FIRST"> </p>
or this:
<p class="x2TTLPG-AU"><br /></p>
I'll next experiment as you recommend with Tweak, or adding styles (after a little more reading).
I have never encountered the added-spacing-from-sideloading problem before, either with ePub or AZW3. I suspect that there is something unique to Kobo's firmware to cause this and perhaps I should investigate further in the appropriate sub-forum.
Thank you for the very helpful info regarding margin-top/bottom in em (which I always use) vs. left/right in pt. I was not aware of the relationship with the zoom level.
Thank you again for your help -- it is greatly appreciated.