It sound like an interesting nut to crack.

I see their are some new relative units on the way for height and width, extending vh and vw. There is large: lvh and lvw. Small svh and svw. The interesting one is dynamic: dvh and dvw. My understanding is that these dynamic values will detect where device UI takes up part of the viewport and adjusts the design to fit so where using 100vh, it won’t get cropped on mobile; you will still see all elements in a layout that are intended to be seen.

It does sound that working designs down to mobile size will become easier to accomplish. At the moment most themes and Wordpress offer three breakpoints based on desktop, tablet (portrait) and mobile. This never seems enough. I’ve seen some themes offer up to 5 viewports, but there are always those edge cases where you have to squeeze in another media query. Something a bit more dynamic would be welcome.