Browsers will show default system font unless you provide fallbacks in your stylesheet. For many people this is something like Times New Roman or Helvetica depending on your OS.
It’s called a font-stack.
Basically a list of font’s that are can be potentially used (in order). if the first font isn’t available browsers move to the second font in the stack.
People generally go with alternate fonts that look similar to the one they really want. example. I might want Libre Franklin as my font, but if it’s not available the fallback might be Helvetica. and then Arial and so Sans.
The font for what I described above looks like:
font-family: "Libre Franklin", Helvetica, Arial, Sans;