Should a UX Designer design in Pixels, Ems, or another unit of measurement?

I'm getting reading to set up my wireframe file for a responsive site. But after reading more about CSS pixels vs. pixels vs. ems, I can't decide how best to design the site and then communicate the design with the development team. This is particularly perplexing when it comes to fonts sizes.

And now with the emergence of Quad HD screens, I'm really not sure how to structure my file and then communicate the sizes of things with the developers. Thoughts?