Well, if you just change the em to pt in your css the text will show up identical in linux and windows since that is a pixel-based size definition much like px which would also work here.
Just a though since the current way you're doing it will also look different if for example someone using windows with a larger font-size set in their browser due to eyesightproblems stumbles upon your website. And hey, you'd think that a few of your future clients might actually be older people with little knowledge of computers and possibly worse eyesight due to age![]()