Bingo! That works. Research assistance provided by son with iPhone.
What devices does this work with? webkit doesn't seem to read "resolution" at all, although mysteriously Chrome does. (So does Camino, meaning that the mozilla family has understood it for ages.) I looked it up to confirm direct observations.
I have one individual page that I use as the acid test for responsiveness, because it's parallel translations and is therefore coded as a three-cell-wide table. (This is much, much cleaner and simpler than slathering the whole thing with named divs.) On small viewports it's supposed to change to block display-- one after the other instead of side-by-side-- so it was an unpleasant surprise to find that my hypothetical iPhone visitor would be faced with table cells at, uhm, around three-quarters of an inch wide.