Been there (my first HTML was in 2000), when CSS had not been invented.
Take time and get your head wrapped in the CSS concept. For most ebooks, it is really simple. I learned by reversing early Baen Webscriptions.
Basically: design IT. Control IT (from a single place. The CSS). Use (reference) IT through out your document.
Code:
/*CSS most simple */
body {}
h3 {}
p {}
/*add if used in your doc */
sup {}
sub {}
blockquote {}
You put your control attributes inside the { } for the tag level item.
Need an exception to the basic rule. add
class="except-name" to the tag on the document
the CSS gets an entry (these always start with/have a dot in the CSS )
BTW both Calibres and Sigils Editor include a
Code Inspector
Calibre you turn it on from the Editors view menu, It will appear at the bottom, and may need to be resized to be of use as it does not seem to
detach. Sigils version: you use the Flashlight Icon to activate the Inspector on the selected block.
Even Firefox has it for decoding web page code (ctrl-shift-C)
The whole point is to control each style (look) from a single place, rater than multiple places.