Why would Arial be placed before Helvetica when using a font-family in CSS?

I think it is not a question of logic or a rule of web type design, as it is a design preference for the appearance of text on page, going from most desired or favored to I will live with it. In early HTML type choice was restricted. The best practice at one time was to specify Verdana, Arial, Times, Georgia. Seems weird today two san serifs then two serif fonts. But at the time Verdana was the typeface designed specifically for screen display, Verdana also being largely cross platform and common so it was called for first generally. This practice was from a time when user had only a limited choice of fonts that came with their computer. So you designed for and used fonts that your user had installed. It is a order of font contingency that was easy to adopt. Make it the way you want it.

Ok, people use it because it is the default in some webpage builder software. The reason it is the default is that back in the day, Macs did not have Arial, only Helvetica. (and in general, Windows had Arial, not Helvetica). Most everyone has both these days, but because Arial was the default Windows sans-serif font for so long (and probably because it begins with "A"), that is the order on the list.  It is not easy to find a machine without Helvetica these days, either, so Helvetica, Arial, sans-serif reverting to Arial is also slim. But it is an old-time acknowledgement of the windows/mac divide, I think. I'm not sure about availability in Linux/Unix/etc. Only when you get to a proprietary font like "Helvetica Neue" are you likely to run into some significant percentage of people that don't have it.