@QuasirandomBefore I settled on the amount of html code, style code, and Calibre for creating epubs I used a range of software programs to make epubs and a range of readers and viewers to examine the results. In the process I found out not every epub converter gave the same results, in fact, most gave different results for some reason I could never fathom. The same was true of the epub readers / viewers. I also learned that you could get some real weird results depending on the font you were viewing the epub in. Now this was about a decade ago, so things may be different, but I now have a system in place that works perfectly and gives the exact results I want.
Mind you, I've found it easier not to use the special character coding because many of the problems I saw back then resulted from the systems not liking the special character coding. By avoiding anything that neds a special character code that starts with & and ends with ; I also minimise the amount of work to do when converting from the word processor to the html, epub, and pdf files I end up with.