That’s the thing: You actually need to save it (including all assets if needed) and then open the local HTML file in the browser or even start a local webserver (IIRC many things aren’t supported from the filesystem due to security reasons. EDIT: Here is the MDN article about that). I wanted to play Trigaea during a long train ride, and I had to download the archive from itch.io and use the Termux CLI app (for which I also develop something btw.) on my phone to start a local webserver so that I could play the game in the browser. That’s not what I call good user experience. The game worked quite well in landscape mode, only some menus were slightly offscreen. Setting the browser to desktop mode fixed that.
I also want my default UI to be mobile compatible, because as I also read today in another thread, people play games on their phones nowadays (myself included, but most mobile games are bad, so I have a select few mobile games and stick to desktop otherwise). I actually prefer reading on my phone (in contrast to on PC), because I can just sit down comfortably with it almost like I would with a book (with a tablet it would probably be perfect).