Operating systems default sans-serif fonts
No doubt that you’ve probably seen a css font-family declaration that didn’t include a specific webfont, but just the sans-serif fallback, or values like -apple-system, system-ui, and BlinkMacSystemFont.
All of those values will return the operating system’s default sans-serif font, with some differences:
-apple-systemworks only for newer MacOS versions (El Capitan and newer) and iOSBlinkMacSystemFontworks only for older Mac OS versions (Yosemite and older)system-uiworks for all operating systems, but there’s one notable exception: it doesn’t work in the Firefox browser. Firefox doesn’t support this value, so your text will be displayed in the default fallback which is a serif fontsans-serifwill work in all browsers and operating systems, but should be only used as fallback, because in some cases operating systems have quirks like different sans font set for web than in its interface
Whether you use a custom font or not, the recommended complete css font-family declaration is:
font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Ubuntu, "Helvetica Neue", Oxygen, Cantarell, sans-serif;
You can safely omit Oxygen and Cantarell since they are targeted to certain Linux versions only. Roboto, Ubuntu, sans-serif will be ok for Linux distros.
Default sans-serif font in MacOS
Please note that — although for interface San Francisco is the default sans-serif font — for web -apple-system will need to be used, as the sans-serif fallback will display Helvetica instead of San Francisco. (this is one of the quirks I mentioned earlier)
Default sans-serif font in Windows
The difference between Microsoft Sans Serif and MS Sans Serif is that MS Sans Serif was a bitmap/raster font available in 8, 10, 12, 14, 18, and 24 sizes and Microsoft Sans Serif is a TrueType scalable font.
MS Sans Serif was based on Helvetica and in all versions up to Windows 3.1 it was called Helv.
Default sans-serif font in Linux
Many Linux users re-configure defaults to other fonts like Inter UI or others. So it’s almost impossible to accurately mention defaults, except for the Ubuntu distro which has its own font as default. Red Hat also has commissioned its own fonts, Red Hat Display and Red Hat Text.