code: public Object setUserData(String key,
                                Object data,
                                UserDataHandler handler) {
          return getOwnerDocument().setUserData( key, data, handler);
      }
docs: Associate an object to a key on this node. The object can later be
  retrieved from this node by calling <code>getUserData</code> with the
  same key.
  @param key The key to associate the object to.
  @param data The object to associate to the given key, or
  <code>null</code> to remove any existing association to that key.
  @param handler The handler to associate to that key, or
  <code>null</code>.
  @return Returns the <code>DOMObject</code> previously associated to
  the given key on this node, or <code>null</code> if there was none.
  @since DOM Level 3