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.
A browser is specifically designed for rendering HTML and does it better than anything else.
However, if you must, you can use something like pandoc to convert the HTML to something else, like a PDF or an Office document.
An alternative is to use a text only browser like lynx.
The ides that a browser is overkill is only true if you don’t already have one installed and only if you’re happy to put up with half baked rendering.
So … use a browser.