Is it possible to use Helvetica for text on your site?
Yes.
You can specify any font you like, but if the people reading your site don’t have it installed on their computer, they won’t see it in that font.
You must be working from a Windows machine. Helvetica is not provided on Windows computers, and isn’t licensed by any Windows program I know of for use. You’ll either have to buy it for your use or use a fake knockoff version. I’ve seen a few fake Helveticas for Windows and they don’t work well at all.
Using it in your font stacks is fine, as long as you provide a fallback:
p {
font-family: helvetica,arial,serif;
}
And definitely put Helvetica in front of Arial.