Because Helvetica looks so bad on Windows, a good compromise for a CSS font stack is something like:

  font-family: "Helvetica Neue",Arial,sans-serif;

Helvetica Neue (a minor reworking of Helvetica from 1983) is a Mac system font, but few Windows PCs have it. So Mac users get nice fonts and everyone else gets Arial.

Many CSS font stacks list Helvetica after Arial, but that is silly because nearly all computers (like 98%) have Arial, so Helvetica will never be selected. :(