Tip 1: Don’t specify Helvetica or Arial over sans-serif. Typically the default sans-serif is a Helvetica clone anyway, meant for that system. For a long time Mac default was Helvetica and Windows default was Arial. There’s very little point in specifying. Just use sans-serif as your baseline fallback.

Tip 2: Specify typical fallback fonts, even with a web-font so that you get coverage for glyphs like emoticons which may not be in your web font.