@meme, Thanks for answering.
Re: your suggested Prefs... I seem to vaguely remember from an earlier Sigil version (5.3???) that one had to have Tidy disabled in order to guarantee avoiding those extra sgc-n css classes appearing in each html file in <style>...</style> tags (possibly with different settings in each file). I want to avoid these at all costs. Will your recommended Prefs prevent me getting into this kind of trouble?