I'll take a stab without seeing an example.
This is the correct:
font-family:"Trebuchet MS", Helvetica, sans-serif;
This is wrong:
font-family:Trebuchet MS, Helvetica, sans-serif;
The browser would make the font family TrebuchetMS, which of course doesn't exist.
Add the double quotes and it will pass validation and render correctly! I hope that helps and I am understanding you question properly.
