[...] The font stack I've used is as follows:
font-family:"Gill Sans", "Gill Sans MT", GillSans, Calibri, "Trebuchet MS", sans-serif;
As suggested by
this site
They say it's a safe stack to use "The Gill Sans stack should work on most all computers. Gill Sans comes on all Macs and Gill Sans MT is installed with Microsoft Office, and Calibri (which is a good stand-in for Gill Sans) is one the core Vista fonts and is installed with both Office Windows and Office Mac. And lastly, if all else fails, use Trebuchet."
So hopefully it'll be alright