title: CSS [custom?] attributes
body: <pre><code>radio[pane] {
   list-style-image: url("jar:resource:///chrome/classic.jar!
           /skin/classic/browser/preferences/Options.png");
  }
  
  radio[pane="prefpane-appearance"] {
   -moz-image-region: rect(0px, 32px, 32px, 0px);
  }
  radio[pane="prefpane-appearance"]:hover,
  radio[pane="prefpane-appearance"][selected="true"] {
   -moz-image-region: rect(32px, 32px, 64px, 0px);
  }
  </code></pre>
  
  <p>Can anyone explain a syntax of this css, particularly what is <code>pane</code>.. I couldn't find such attribute for radio element in context of XUL. So I guess it's some custom attribute? If it is, then how it is evolving through the lines, first declaration, then several assignments? It has also <code>selected</code>, which means can have multiple custom attributes?
  How can those attributes be used later?</p>