title: How to detect which one of the defined font was used in a web page?
body: <p>Suppose I have the following CSS rule in my page:</p>
  <pre class="lang-css prettyprint-override"><code>body {
      font-family: Calibri, Trebuchet MS, Helvetica, sans-serif;
  }
  </code></pre>
  <p>How could I detect which one of the defined fonts were used in the user's browser?</p>
  <p>For people wondering why I want to do this is because the font I'm detecting contains glyphs that are <em>not</em> available in other fonts. If the user does <em>not</em> have the font, then I want it to display a link asking the user to download that font (so they can use my web application with the correct font).</p>
  <p>Currently, I am displaying the download font link for all users. I want to only display this for people who do <em>not</em> have the correct font installed.</p>