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-system works only for newer MacOS versions (El Capitan and newer) and iOS
• BlinkMacSystemFont works only for older Mac OS versions (Yosemite and older)
• system-ui works 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 font
• sans-serif will 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

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.

Default sans-serif font in iOS

Default sans-serif font in watchOS

Default sans-serif font in Android