[ssmoot]

I think the future-proofing was just a "why not?" bonus. The real goal was just to render with system fonts so the page felt as native as possible and they got to take advantage of all the person-hours put into the choice by the OS vendors.

But the fonts do change on occasion, so it makes sense to me, for forwards and backwards compatibility, you'd ideally just want to be able to say "render whatever font the rest of the system is using".