I often see the websites using font \"Helvetica Neue\". Is this font safe to use, like eg. Arial? Or do the browsers have trouble rendering it or not many machines have this fon
Helvetica Neue is a paid font, so you shouldn't @font-face it, as you'd be freely distributing a copyrighted font. It's included in Mac systems but not in windows/linux ones, so yes, plenty of your users wont have it installed. Anyway, you can use 'Arial Narrow' as a windows substitute, which is it's windows equivalent.