The current CSS design for Ask Different puts Arial first.
I think we should change this CSS to put San Francisco first, without having to license anything or anything like that. Here is my CSS code:
body { font-family: -apple-system, Helvetica Neue, Arial; }
The only problem with this is that -apple-system is only supported on Apple devices and Safari. However, it can default back to Helvetica Neue.
I also created a TamperMonkey script with this same behavior
font-family: system, -apple-system, BlinkMacSystemFont, "Helvetica Neue", Helvetica, "Lucida Grande", Arial, sans-serif;