I hope you're not implying that web sites ought to specify the font for body text. Headings, sure. But if you want people to stick around and read your content, better let them read it in the form they're comfortable with.
Some web sites do not specify a font or specify only (eg) Arial, which is not available on some platforms.
Well, if they didn't want to be different they wouldn't be using Linux in the first place :P
btw Arial is perfectly safe for Mac. The font-stack area at codestyle dot org says 98.89% Mac vs. 99.90% Windows, which is not a huge difference. (Even Linux runs about 2/3 if you can believe their numbers.)
:: insert painful description of potentially interesting blog site written in-- I am not making this up-- eleven-point #4B5D67 monospace ::