It’s a difficult problem - eventually, we’ll implement online the same system we have locally for fetching fonts from Google, but until that’s up and running we have a choice between Open Sans (which is from Google, so will work locally regardless of operating system and installed fonts, but doesn’t work online yet) or Arial (which works online as it’s “web safe”, ie packaged with most browsers, but isn’t necessarily going to be installed when running locally). We opted for Open Sans as the default as it will eventually work online and it’s freely available from Google Fonts.
Here’s a list of web safe fonts:
If you choose the most readable of these it will work on the vast majority of browsers. We’re growing our JS team too, and online fonts are a priority, so keep an eye out in the near future!