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.
FWIW “just displaying html” is one of those can of worms type problems that looks easy enough to start making but actually impossible to do fully correct.
You want something thats doing a WebView, so if not a Firefox then maybe one of the lighter weight ones like Falkon or Gnome Web etc.