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.
Do you just want to see the text content of a HTML file? - a text editor
Do you want most, if not all, HTML tags to be rendered as pretty graphical shapes?
Do you want the text have proper fonts?
Styles? You need something to parse CSS files.
What about dynamically generated content like ten smiley faces? You need a JavaScript engine.
Do you also want to see iframes? You need it to be capable of sending XHR requests.
What if it references to a piece of WebAssembly?
It’s way more complicated than you anticipated.