The site prefers Segoe UI, if absent (on any mobile device or non-Windows desktop) defaults to Helvetica Neue or Helvetica.
The difference in fonts is quite dramatic, which doesn't make for a consistent experience across devices.
I probably prefer Helvetica over Segoe UI, but my larger point is that there should be just one font, whichever you choose.
Put differently, the default font should be one which will be supported on the majority of devices.
As it stands now, any Windows computer gets Segoe UI, any Mac/iPhone/iPad displays Helvetica Neue (unless you manually install Segoe UI on the Mac, then will also display Segoe UI), and Android appears to default to Roboto.
Find one font that will work across devices, and use it.