@Liudprand We are here to try and help each other.
MR has different sections that focus on topics for writers or publishers like yourself.
Calibre is a tool originally aimed at
storing books and making them compatible with many devices. (It grew way past that

) It is NOT
intended as an authoring tool.
I use the Editor as a touchup tool rather than trying to make Conversion (of the same or different format) do the lifting blind(ish)/trial & Error. I find I can use the Editor to find and debug most visual oddities faster than it took you to log onto MR and post a coherent query on how to get
convert to do it.
Most of the code is basic HTML (not the full set used on Web Pages). SIMPLE. There are a few tutorials here on MR.

(FWIW I hardly knew anything about decent HTML and no CSS when I first came here. I thank the many folk that helped me along the way)

Do as much as possible with the CSS
file, avoiding inline STYLE= as much as possible. Did you notice I underlined FILE? That is because you
could put that code at the top of each (x)HTML file. But that leads to inconsistencies between chapters

1 (CSS) file to rule ALL of your book make tuneups easy.
MR Writers corner is more aimed at the craft of writing and marketing. Great folk, who have been in the game for a while. And a number of PRO's (both in formatting and Writing) provide input to polite queries.