That's a great idea in theory, but bear in mind that (in most target audiences) few people will know how to change the default typeface and even fewer will bother. I'd be willing to bet that 95 percent of your visitors will be seeing Arial, Helvetica or Roboto.

I'm not trying to dissuade you, just make you aware (if you weren't already) that you're making a choice by not making a choice.

In CSS you want to use the special name 'sans-serif' (note the dash) to indicate the default user configurable sans-serif font.