By now the spec is following:

    <p>
      <u>Hiding a modal dialog</u><br>
      When the modal dialog that currently has focus is hidden, its owner, if
      unblocked, may become the active window&#151; however, the actual behavior is
      unspecified and platform-dependent. If the modal dialog to be hidden doe not have
       focus, the active window remains unchanged.
    </p>

We should mention that the previous active window may become the active window in case of a modal dialog without owner.