Sigil is an EPUB
editing (code) tool. By that, I mean it maintains the required structure and other attribute that are common to EPUB.
OTOH (general) Formatting is usually done with a WYSIWYG Creation tool.
Sigil is great for CSS touchup, inserting Images, Spell checking and TOC creation (from headings).
It includes a synchronized Preview (no longer has the ability to type into that view), with a code inspector (what the heck code is making that section look like a
?
) if you need to unravel a cascaded style blooper.