The standard fonts sets can seem limiting if you’re more used to print work, but in fact they’re not at all bad for setting paragraphs of text. Georgia is a particularly clear serif font at small sizes, as is the sans-serif Verdana. Both of these were designed by Matthew Carter for Microsoft, and are excellent choices when legibility is paramount. Trebuchet MS is another good sans-serif font for body text, and it sets smaller than the rather wide-bodied Verdana. Arial is actually not too bad, although it isn’t quite as well sculpted for use as on-screen body text. The rest of the web-safe serifs and sans fonts - meaning Helvetica, Times and Times New Roman, and Geneva - are safe in the sense that they’re good as backups that are generally going to be available, but they’re not as good in terms of clarity.