[Quoth]

I agree with what theducks and DNSB write.

If you do a conversion (and epub to epub works), then select 'Look and Feel' > 'Styling' and rather than Extra CSS, at the bottom there is 'Filter style information', 'Select what style information you want to remove'
add bold text at
'Other CSS Properties:' [white-space, line-height]

There is some validity on web pages and some for fixed layout, but regular ebooks shouldn't have either of those.
Any extra white space should be only by padding or margin properties which this won't touch.
The line-helght is actually automatic from font metrics and if not set, the user can change it on the GUI. If it's set the user can't change it and it may be stupid.

So far those are the only two things I find safe to do on every ebook. I do extra stuff on files from Gutenberg.
It's in my calibre default for every kind of conversion.

Some ebooks will need manually edited and it usually doesn't long with the 'Edit Spans & Divs' plugin (and more) and sometimes regex fine & replace on the pages. Use 'Find' first a few times to be sure!
You can go to a paragraph and 'Control' mouse right click on the class name at the opening tag.

Knowledge of HTML and CSS is needed, but those in reflowable epub2, or awz3 / mobi etc converted to epub2 are not hard to learn.