An element that displays only one of its children at a time. The selectedIndex attribute determines which child is displayed.
More information is available in the XUL tutorial.
<deck selectedIndex="2">
  <description value="This is the first page"/>
  <button label="This is the second page"/>
  <box>
    <description value="This is the third page"/>
    <button label="This is also the third page"/>
  </box>
</deck>
selectedIndex| Inherited from XUL element | 
selectedIndex-1 to this property, all items will be deselected. Returns -1 if no items are selectedselectedPaneldeck element. Assigning a value to this property will modify the selected panel.| Inherited Properties |