Website font is not standardized across platforms
For example, on my Mac the system font is used, and on my PC Segoe UI is used. It's subtle, but I think standardizing the font is important to the "brand" identity of the site. Even the "Tildes" text logo at the top of the pages changes based on the system.
There are plenty of free open-source fonts available on Google Fonts, like Noto Sans: https://fonts.google.com/specimen/Noto+Sans