CSS: Is Helvetica the default 'sans-serif' font on Mac and Arial the default sans-serif font on Windows?

Question

I have a lot of CSS that does the following:

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

It my understanding that Helvetica is the default sans-

Answer 1

What do you want to happen on platforms that have

• Helvetica or Arial installed and
• a default sans-serif font that is neither of those?

Or asked differently: do you always prefer Helvetica or Arial over the default, if they are installed? If you prefer the default sans-serif font in all cases, why mention those two at all?