> MS Word is properly great if you only use the custom style rules (basically CSS classes) at the paragraph level, and never directly apply styling (basically inline styles) except for super basic stuff like making a word italic

MS word also has character styles (like a CSS style on a <span>). IMO you should use instead of bold or italic.

(There are three more types of styles: linked, table and List. See https://office-watch.com/2022/word-five-types-styles/)

I'm aware, but Word's notorious "I clicked one button and it ruined the formatting of my entire document" stuff doesn't happen if you mark a word as italic or bold here and there in the middle of a sentence. The whole point of only using the style rules is to prevent it doing that.

But yeah for layout, ie headings and the likes, only ever use the styles, never "bold, bigger bigger bigger". Don't touch the line spacing button, etc etc.

IMO Word could do with a mode where those buttons are simply hidden. Want a bigger, fatter heading? Edit the heading style. There's no other way.