Have you ever sent a beautiful document to a friend, only to find out that they didn't have the right font to view it?

The same applies to web pages: if we use a not-so-common font family, some folks might not have it. That's why it's a great idea to have a backup font family.

<html>
 <head>
  <style>.headline {font-family: "Akzidenz-Grotesk","Helvetica", sans-serif;font-size: 1.5em;font-weight: bold;}</style>
 </head>
 <body><h1 class="headline">I shot the serif</h1></body>
</html>
]]>

Nice! The web browser will try the first font family; if it's unavailable, it'll try the second. In the end, it'll use the default serif or sans-serif font family.

That doesn't look right, does it?