sorry, but You don't seem to understand HTML/CSS
You don't NEED "class= " for HTML tags. None of the responses are wrong, just different. But they all have 1 thing in common K.I.S.S.
For years, Baen (you've heard of them?) shipped ebooks with 3 tiny CSS
1 of those was even blank (the conversion scripts required it to be included. so it was empty rather than break other stuff)
Here is the BASE CSS I use all the time. 2 entriers

(for simple TXT files that I import)
I sometimes add a Chapter Number (CN) treatment, Opening Paragraph treatment (FP)