[bogwog]

I wonder if you could use CSS media queries to collect additional data about device characteristics, by linking to different URLs at different breakpoints?

Like a breakpoint for portrait mode would set the background of something to an image that's just a 1x1 pixel. So when that resource is accessed, you know the request came from a device in portrait.

I guess it depends on how different browsers decide to access resource urls in CSS files. If a browser just downloads everything first, and then processes the media queries, then it wouldn't work.