I saw the "Summer 2021 HDX development" thread, but it didn't deal with this issue.
Many phones, probably all high-end ones, have something around 4K resolution. But they don't have super resolution to display a lot more stuff on the screen, they have it to resample from different resolution without visible artifacts. The screens are still small, and can't show very much.
Many web pages know how to adapt to this situation. Travel Mapping would be easier to use on these phones (I have a Pixel 3XL) if it did, too. When I open it, I see basically what I'd see on my 4K desktop monitor, if I used portrait orientation. On the phone, the title bar (of the page, not the browser) is extremely tiny, so the checkboxes are very hard to check, taking me ten or twenty taps before it lands in that tiny square. The route highlighting is also extremely thin, and almost impossible to select by touch.
If I manage to squeeze my fingers into the title bar, and unpinch, I can zoom in on the image. (This is with Chrome on Android.) But that isn't zooming the page within the browser window, it's zooming the entire window within the display. Now I've got two things I can scroll, depending on where I touch. If I drag the map, I'm scrolling the map, which is actually much larger than what I'm looking at. If I can get to the stuff outside the map, then I can scroll the magnified browser window within the actual screen. If it updates the display, it's loading much more data than I can see.
Needless to say, this does not make for a very user-friendly experience. I'm commencing a cross-country trip, and would like to use the phone to see where I might go to find untraveled routes while I'm driving. I'll be using my laptop to record my travels each evening, but just being able to use the mapview page more easily on the phone would have been immensely useful. (Not that I expect this would be doable before my trip is done.)