I'm using in CSS:
font-family:Arial,Helvetica,"Nimbus Sans L",sans-serif;
font-size: 11px;
The dimensione of the text in Windows XP/Vista/7 on Firefox/IE/Safari is different from OSx on Firefox/Safari and Ubuntu on Firefox.
Anyone know why this happens and has a solution for fixing it?
yes I had found this exact same problem,
this is because mac actually very slightly bolds every single font, sometimes noticably sometimes not, but enough to make a different to you.
do a google search for "css hacks" and have a root around or google saearch specifically "css font hacks"
alternatively try some obvious things like setting font-weight to none, or setting line height to 1.5 em:
let me know if this helps