This is the generic "sans-serif" font

This is the generic "serif" font

This is the "Arial" font

This is the "Courier" font

Note that the above lines are a class attached to a p tag.
The below lines are attached to a div tag.
Looking at the source code shows the difference.

This is the generic "sans-serif" font

This is the generic "serif" font

This is the generic "Arial" font

This is the generic "Courier" font