I just tried to open the HA interface on a mobile device without the companion app, person does not like to install app please do not ask.
But I only get a desktop version of the page. So everything works but is really small. So I was wondering if that is a bug or known limitation. I do not see a reason why it should not work on a mobile devices without an app.
Mine looks just like the HA app - Lovelace is responsive. Make sure you don’t have “Desktop Site” ticked on your browser. I am using Chrome on Android.
It properly renders on our (Android) mobile devices, in a variety of browsers. We don’t use/need the app.
The only caveat is that sometimes entity names are truncated more than on the desktop view, so I’ve had to rename a few things so that the distinctive parts of the name come first.