What is a font fallback and how do you specify one?

What will be an ideal response?

You can specify a list of alternate fonts using a comma as a separator. The browser attempts to load each successive font in the list. If no fonts match, the browser falls back to the default font. The following code tells the browser to use Arial; if Arial is not present, the browser uses Helvetica.p {font-family: arial, helvetica;}This font substitution string produces a sans-serif font on PCs that have Arial installed and on Macintosh computers that have Helvetica installed. To further ensure the portability of this rule, add a generic font family name to the list, as shown in the following rule:
p {font-family: arial, helvetica, sans-serif;}
This rule ensures that the

element is displayed in some type of sans-serif font, even if it is not Arial or Helvetica.