The font-family and font properties accept a list of generic font family names:
serif, sans-serif, cursive, fantasy and monospace.
If you wrap these generic family names in quotes, the browser will not interpret them as generic font family names,
but will instead look for a font by that name (e.g. will look for a "sans-serif" font) -- which is almost never what
you want.
font-family: 'Arial', 'sans-serif';
font-family: 'Arial', sans-serif;