No CSS, leaves the work at the mercy of the device DEFAULTS
H1 default on a 6" device is limited; 'Chapter 42' probably won't fit.
Typically I spend less than 5 minutes tuning the CSS.
Calibre and Sigil both have a Jump to (style sheet location) for the class= selected.
OTOH EPUB 3 sometimes Nests so many tags, that finding the one that is making you unhappy, can be a challenge.
The 'Inspector' tool (part of the Editor) can help track things down