I have wondered many times.
Of course I can always use a browser but it’s overkill.
The same goes for yad or zenity, they pull in webkit which is a full-fledged browser engine, and at least yad does not have an offline mode.
I just want to look at some local HTML (incl. images) & CSS styling.
Maybe an e-book reader like KOReader? https://flathub.org/apps/rocks.koreader.KOReader
Plenty of them support local HTML.
probably most of them do. epubs are mostly html after all.