Is it necessary to test css styles with QUnit?

I'm now developing a web app with EmberJS and meanwhile I use the build-in QUnit framework to test it, I got a few questions about test and styles listed below.

1 Is it necessary to test css styles? I mean write test case instead of visually check in browser.

2 Should I use px or rem in the style sheet? I used to use rem for responsive design and HiDPI devices, but recently I found that media may do the job, and there are voices that supports px over rem. Like this http://ift.tt/1CeSVLt

3 Something really, really wired... I set a span's font-size to be 15px, but QUnit test results turns to be 18px (html base font-size is 16px, manually set). Just very simple DOM structure, no other font-size rules attached.enter image description here

Thanks in advance!