This explains the benefits:

http://blog.cloudfour.com/the-ems-have-it-proportional-media...

Here's some boilerplate code:

https://github.com/johannaruiz/propotional-mqs

Make sure you set the viewport correctly:

http://blog.javierusobiaga.com/stop-using-the-viewport-tag-u...

as an incorrect viewport can cause issues with some mobile browsers:

http://www.quirksmode.org/blog/archives/2013/11/a_day_in_the...